Circadian Rhythms Study Guide

Circadian Rhythms - Study Notes

Introduction to Circadian Rhythms

• Definition: Circadian rhythms are repetitive biological events with a period length of about one day.

• Properties:

• They oscillate continuously, even in constant environmental conditions.

• These rhythms are roughly 24 hours long and can be synchronized by external light/dark cycles.

Nobel Prize in Physiology or Medicine 2017

• Awarded to Jeffrey C. Hall, Michael Rosbash, and Michael W. Young for their discoveries on biological rhythms.

• Their work elucidated how living organisms synchronize their internal clocks to the Earth's rotation.

Slide 3: Circadian Rhythms and the Nobel Prize Discovery

Key Points:

• Nobel Prize was awarded to Jeffrey C. Hall, Michael Rosbash, and Michael W. Young for uncovering how biological clocks function.

• Their work demonstrated how circadian rhythms—internal biological clocks—synchronize with the Earth’s 24-hour cycle.

• Circadian rhythms regulate daily physiological processes like hormone levels, sleep, body temperature, and alertness.

Explanation of Visuals:

• The circular diagram shows key biological events across a 24-hour cycle:

  • Cortisol peaks in the morning.

  • Highest alertness and physical performance in the afternoon.

  • Melatonin rises in the evening to promote sleep.

Glossary:

• Circadian rhythms: Biological cycles of about 24 hours regulating sleep, metabolism, and other bodily functions.

• Melatonin: A hormone that helps regulate sleep-wake cycles.

• Cortisol: A hormone associated with stress response and wakefulness.

Key Takeaway:
Circadian rhythms are natural, internal processes influenced by Earth’s light-dark cycle, critically important for regulating daily physiological functions.

Slide 4: Why Care About Circadian Rhythms?

Key Points:

• This slide introduces the central question: What is the relevance of circadian rhythms to our lives?

• Invites the audience to reflect on whether these rhythms affect their daily activities, health, and well-being.

Explanation of Visuals:

• The slide is primarily textual and conceptual, posing a thought-provoking question for class discussion or deeper exploration.

Glossary:

• (Not applicable – no specific terms on this slide.)

Key Takeaway:
Understanding how circadian rhythms impact our lives helps frame their biological and medical importance.

Slide 5: Circadian Rhythms in Daily Life

Key Points:

• Circadian rhythms influence daily behavior and physiology in both humans and animals.

• These rhythms affect:

  • Sleep-wake cycles.

  • Cognitive function (mental alertness).

  • Daily behaviors and response to light.

• The rhythms are embedded in how we experience time and organize daily activities.

Explanation of Visuals:

• Clock and brain imagery symbolize the interaction between time, brain activity, and external light.

• Additional images (e.g., rotating behavior video or animal motion) imply observable rhythmic behavior in nature.

Glossary:

• Sleep-wake cycle: The daily pattern of sleepiness and alertness governed by circadian rhythms.

Key Takeaway:
Circadian rhythms shape how we experience time and control many aspects of our everyday functioning and behavior.

Slide 6: Historical Discovery of the Circadian Clock (1729)

Key Points:

• The circadian clock was first observed in 1729 by Jean-Jacques d’Ortous de Mairan.

• He discovered that plants open and close their leaves even in constant darkness, suggesting the existence of an internal biological clock.

• This was the first evidence that biological rhythms can persist without external light cues.

Explanation of Visuals:

• Experimental setup shows plants placed in both light-dark and constant dark conditions.

• Plants maintain a rhythmic behavior even without light, proving the presence of an internal rhythm. So it was not mainly about the light that was the trigger.

Glossary:

• Endogenous rhythm: A biological rhythm generated internally, not dependent on external cues like light.

Key Takeaway:
- Circadian rhythm is not only in humans, but also in plants.

The discovery of circadian rhythms in plants in the 18th century revealed that biological clocks operate independently of environmental light.

Slide 7: Foundational Work – Pittendrigh and Aschoff (1960–1965)

Key Points:

• Colin S. Pittendrigh (USA) and Jürgen Aschoff (Germany) are regarded as founders of modern chronobiology.

• They were the first to define key properties of circadian rhythms in systematic experiments.

• Their work laid the groundwork for understanding how internal clocks function across species.

Explanation of Visuals:

• Photographs of Pittendrigh and Aschoff with short descriptions of their major contributions:

  • Pittendrigh: Focus on environmental entrainment of rhythms.

  • Aschoff: Defined conditions under which circadian rhythms persist.

Glossary:

• Chronobiology: The scientific study of biological rhythms and their mechanisms.

• Circadian rhythm: Biological processes that follow an approximately 24-hour cycle.

Key Takeaway:
Pittendrigh and Aschoff established the core principles of circadian biology, shaping how we study biological clocks today.

Slide 8: Definition and Core Properties of Circadian Rhythms

Key Points:

• Definition: A circadian rhythm is a biological process that repeats roughly every 24 hours.

• Core properties:

  • Persist in constant conditions (e.g., darkness) with ~24-hour period.

  • Endogenous: These rhythms originate internally, not requiring external cues.

  • Internal clocks run at about the same speed at different temperatures, even in cold-blooded animals and plants.

  • Occur in a wide range of organisms: humans, animals, and plants.

  • Can be synchronized (entrained) by external cues like the light-dark cycle.

Explanation of Visuals:

• Text-based slide with highlighted terms for emphasis.

• Clearly defines what makes a rhythm “circadian” and how it is distinguished from responses driven solely by environment.

Glossary:

• Endogenous: Originating from within the organism.

• Entrainment: Synchronization of the internal clock to external signals like light.

Key Takeaway:
Circadian rhythms are self-sustained, internal cycles that align with external cues but continue independently in constant conditions.

Slide 9: Circadian Rhythm Parameters – Graphical Representation

Key Points:

• Important characteristics of circadian rhythms:

  • Period length: Time it takes to complete one full cycle (typically ~24h).

  • Amplitude: Strength or intensity of the rhythm.

  • Phase: Timing of a specific point in the cycle (e.g., peak alertness).

  • Phase shift: Change in timing, often in response to environmental cues.

Two types of conditions:

Entrained Conditions: Clock receives synchronizing stimuli from outside

Free-Running Condititions: Clock is kept isolated under constant conditions.

Possible Stimuli from outside are:

• Sunlight

• Food Supply

• Exercising

• etc.

Explanation of Visuals:

• Graph showing oscillations of two circadian rhythms:

  • Illustrates differences in amplitude, period, and phase.

  • A phase shift is shown where one rhythm is delayed or advanced relative to another.

Glossary:

• Free-running condition: Circadian rhythm measured without external cues (e.g., constant darkness).

• Phase shift: Adjustment in timing of the rhythm due to stimuli like light.

Key Takeaway:
Circadian rhythms are characterized by their period, amplitude, and phase, all of which can adapt or shift in response to environmental signals.

Slide 10: Where Can We Find Circadian Clocks?

Key Points:

• Circadian clocks are present across all domains of life, from simple bacteria to humans.

• They are found in:

  • Eukaryotes: Animals (mammals, insects), plants, fungi.

  • Prokaryotes: Certain bacteria (e.g., Synechococcus).

• This widespread occurrence highlights the evolutionary importance of internal timekeeping systems.

Explanation of Visuals:

• Evolutionary tree showing a wide range of organisms with identified circadian systems.

• Red text labels indicate organisms known to have characterized circadian clocks.

Glossary:

• Eukaryotes: Organisms with complex cells containing nuclei.

• Prokaryotes: Simpler organisms without a nucleus (e.g., bacteria).

Key Takeaway:
Circadian clocks are an evolutionarily conserved feature, present across nearly all branches of life, reflecting their critical biological function.

Slide 11: First circadian gene discovered in Drosophila melanogaster (1971)

Key Points:

• The first circadian gene ("period" or per) was discovered in fruit flies (Drosophila melanogaster) in 1971 by Konopka & Benzer.

• This gene was found through genetic studies showing how mutations in per disrupted circadian behavior.

• Drosophila is used due to its genetic tractability, simple brain, and observable circadian-controlled behaviors (e.g., activity cycles).

Explanation of Visuals:

• Photos of the researchers (Konopka and Benzer) and a cartoon Drosophila with labels showing:

  • Simple brain (ideal model).

  • Genetic accessibility for mutation studies.

Glossary:

• Drosophila melanogaster: A model organism (fruit fly) commonly used in genetic research. as it has a higher complexity, more orientated on human genome.

• Circadian gene: A gene whose function is to control internal biological rhythms, such as sleep-wake cycles.

Key Takeaway:
The discovery of the per gene in fruit flies laid the foundation for molecular circadian biology by linking genes to rhythmic behavior.

Slide 12: Circadian research in Drosophila

Key Points:

• Drosophila are exposed to light-dark cycles in laboratory tubes to study circadian rhythms.

• Behavioral readouts (e.g., activity at dawn and dusk) are used to identify rhythmic gene expression.

• Light entrainment is used to synchronize flies to environmental cycles, revealing molecular clock dynamics.

Explanation of Visuals:

• Image of a fly in an activity monitor (a tube setup) and actograms (plots showing daily activity patterns).

• Shows how researchers measure rhythmic behavior in response to light cycles.

🔬 What were the results?

• The flies show clear circadian behavior:

  • Activity spikes at dawn and/or dusk, repeating daily.

  • This means their internal biological clock is synchronized to the light/dark cycle.

• The rhythm continues over multiple days → indicating an endogenous (internal) circadian clock, not just a response to external light.

Glossary:

• Entrainment: Synchronization of internal rhythms to external cues like light.

• Actogram: A chart displaying activity/rest cycles over time.

Key Takeaway:
Drosophila serve as a powerful system for studying how light cycles entrain circadian gene expression and behavior.

Slide 13: Physiological per gene oscillations in Drosophila

Key Points:

• The period (per) gene in Drosophila exhibits daily oscillations in RNA levels, peaking around dusk.

• This rhythmic gene expression is a molecular hallmark of circadian clocks.

• The pattern reflects an internal clock synchronized to the environment.

Explanation of Visuals:

• Graph shows per mRNA levels over a 24-hour cycle:

  • Low in the morning (dawn), rising during the day, peaking in the evening (dusk), and dropping overnight.

Glossary:

• mRNA (messenger RNA): A molecule transcribed from DNA that is used to make proteins.

• Oscillation: Repeated rise and fall in levels (e.g., gene expression).

Key Takeaway:
Circadian gene expression involves daily per mRNA oscillations, forming a core part of the biological clock in fruit flies.

Slide 14: per⁰ mutant Drosophila loses circadian oscillations

Key Points:

• In per⁰ (null or nonfunctional mutant) flies, no rhythmic expression of per mRNA is observed.

• As a result, circadian rhythms are lost—these flies do not show normal daily patterns of activity or gene expression.

• Confirms that the per gene is essential for generating circadian rhythms.

Explanation of Visuals:

• Graph compares per mRNA levels in:

  • Wild-type flies (blue curve): show rhythmic expression.

  • per⁰ mutants (red line): flat expression—no oscillation.

Glossary:

• per⁰ mutant: A fly with a defective per gene, incapable of generating normal circadian rhythms.

• Null mutation: A genetic mutation resulting in complete loss of function.

Key Takeaway:
Mutations in the per gene eliminate circadian rhythms, demonstrating its central role in the molecular clock.

Slide 15: period (per) Gene Mutations and Their Effects

Key Points:

• Mutations in the per gene result in altered circadian periods:

  • WT (wild-type): ~24h rhythm.

  • per^S (short): Shortens period to ~19h.

  • per^L (long): Lengthens period to ~28h.

  • per⁰ (null): Abolishes rhythmic behavior entirely.

• These mutations were first observed in Konopka & Benzer’s 1971 study.

Explanation of Visuals:

• Actograms showing activity rhythms of flies with different per mutations.

• Each mutation leads to a distinct change in circadian behavior.

Glossary:

• per^S / per^L / per⁰: Different alleles (mutations) of the period gene.

• Actogram: A graphical representation of behavioral rhythms over time.

Key Takeaway:
Different mutations in the per gene alter the period or abolish circadian rhythms, highlighting its central role in biological timekeeping.

Slide 16: The Transcription-Translation Feedback Loop (TTFL)

Key Points:

• The TTFL is the core mechanism of circadian clocks in most organisms.

• In Drosophila:

  • The transcription factors CLK (Clock) and CYC (Cycle) activate the transcription of per and tim.

  • PER and TIM proteins accumulate, form a complex, and go into the nucleus and inhibit CLK and CYC and their own transcription, forming a feedback loop.

• This loop creates rhythmic oscillations in gene expression over ~24 hours.

Explanation of Visuals:

• Circular diagram showing:

  • Activation of per and tim genes.

  • Accumulation and feedback inhibition by PER/TIM complex.

  • Degradation of proteins and restart of the cycle.

Glossary:

• TTFL (Transcription-Translation Feedback Loop): A molecular loop where proteins inhibit their own gene expression, generating circadian rhythms.

• CLK/CYC: Activator proteins.

• PER/TIM: Repressor proteins.

• Transcription factors: Transcription factors are proteins that regulate gene expression by binding to specific DNA sequences,

Key Takeaway:
Circadian rhythms are driven by feedback loops in which clock proteins inhibit their own production in a ~24-hour cycle.

Slide 17: Basic Principles of a TTFL

Key Points:

• The TTFL consists of:

  • Activator proteins (green): Promote gene expression.

  • Repressor proteins (red): Inhibit the same genes once enough protein is made.

• This creates a delayed negative feedback loop, which is essential for rhythmic oscillation.

• When the Activator (green) is transcribing the inhibitors, the activators get inhibited

• But as soon as too much inhibitor is transcribed, the activator is less and less presence

• Meaning that now since inhibitor is not being transcribed by the activator, until activator comes back. → and when the activator (green) proteins (transcription factos) are back again, the transcription of the inhibitors start again.

Explanation of Visuals:

• Simplified schematic:

  • Activator (A) turns on the gene.

  • Repressor (R) is produced, feeds back to turn off gene expression.

Glossary:

• Negative feedback loop: A process in which the output of a system inhibits its own production.

Key Takeaway:
The TTFL works by using repressor proteins to inhibit the activators that turn on gene expression, generating self-sustained oscillations.

Slide 18: The Circadian TTFL in Drosophila

Key Points:

• The full Drosophila circadian loop involves:

  • CLK/CYC activating per and tim transcription.

  • PER and TIM proteins accumulating in the cytoplasm, forming complexes.

  • The complex enters the nucleus to inhibit CLK/CYC, stopping transcription.

  • Light signals lead to TIM degradation, resetting the clock.

• This loop creates rhythmic gene expression and behavior.

Explanation of Visuals:

• Full TTFL schematic in Drosophila:

  • Shows protein movement, interactions, and inhibition cycle.

  • Includes input from light as an entrainment cue.

Glossary:

• Entrainment: Synchronization of the internal clock by environmental signals like light.

Key Takeaway:
In Drosophila, circadian rhythms are generated through a feedback loop of transcription and translation, regulated by protein accumulation and light input.

Slide 19: The same genes are important in mice

Key Points:

• Core clock genes like Per are conserved across species, including mice.

• Clock gene mutations in mice lead to loss of circadian behavior, just as in Drosophila.

• This validates mice as a powerful model for studying mammalian circadian rhythms.

Explanation of Visuals:

• Every row represents a day

• Actograms show the difference in activity between:

  • Wild-type mice: exhibit clear 24-hour activity cycles.

  • Clock mutant mice: display arrhythmic behavior (no consistent daily pattern).

• A picture of a running wheel setup indicates how activity rhythms are monitored in mice.

Glossary:

• Clock mutant: Mouse with disrupted circadian gene function.

• Arrhythmic: Lacking regular rhythmic behavior.

Key Takeaway:
Like in Drosophila, mutations in clock genes disrupt circadian rhythms in mice, confirming that these genes are fundamental to biological timing in mammals.

Slide 20: Per genes in other vertebrates

Key Points:

• Circadian rhythms exist in many vertebrate species, including:

  • Fish, amphibians, birds, reptiles, and mammals.

• The Per gene family plays a conserved role in generating circadian rhythms across all these animals.

• This conservation suggests an evolutionarily ancient and essential function of these genes.

Explanation of Visuals:

• Images of different vertebrates, from zebrafish to birds to mammals.

• Fluorescent labeling in brain slices shows Per gene expression in various tissues, including the suprachiasmatic nucleus (SCN) in mammals.

Glossary:

• Suprachiasmatic nucleus (SCN): The central circadian clock located in the brain of mammals.

• Conserved gene: A gene that maintains similar structure and function across different species through evolution.

Key Takeaway:
The Per gene family is central to circadian regulation across all vertebrates, showing that internal clocks are deeply conserved in evolution.

Slide 21: The circadian TTFL in Drosophila and mice

Key Points:

• Both Drosophila and mice use similar Transcription-Translation Feedback Loops (TTFLs), but with species-specific proteins.

• In mice:

  • BMAL1/CLOCK are the main activators (equivalent to CLK/CYC in Drosophila).

  • PER and CRY proteins act as repressors (similar to PER/TIM in flies).

• This highlights the functional conservation of circadian mechanisms between insects and mammals.

Explanation of Visuals:

• Side-by-side comparison of TTFL components:

  • Drosophila (top loop) and mouse (bottom loop).

  • Both cycles show activator proteins initiating transcription of clock genes, which are then repressed by the translated protein products.

Glossary:

• BMAL1/CLOCK: Key transcription factors in the mammalian circadian clock.

• CRY (Cryptochrome): A core repressor protein in mammals.

• TTFL: Transcription Translation Feedback Loop

Key Takeaway:
Despite using different proteins, both insects and mammals share a similar circadian feedback loop design, underlining the conserved logic of clock regulation.

Slide 22: Ten percent of genes are expressed in circadian fashion

Key Points:

• Only around 10% of all genes in an organ are expressed with a circadian rhythm.

• These rhythmic genes vary by tissue:

  • For example, rhythmic genes in the liver differ from those in the heart.

• This implies that circadian regulation is widespread and tissue-specific, influencing many physiological processes.

• Your brain cells have different clocks, your eye cells have different clocks and still it is somehow synchronized.

Explanation of Visuals:

• Heatmaps and radial plots show gene expression patterns over 24 hours in the liver and heart.

  • Peaks of expression vary by gene and tissue.

• Indicates that timing is an important layer of gene regulation.

Glossary:

• Heatmap: A visual representation of data where values are represented by color.

• Circadian gene expression: Genes that turn on/off in a rhythmic pattern aligned to the 24-hour cycle.

Key Takeaway:
A significant portion of the genome is under circadian control, with gene expression rhythms tailored to the specific functions of each tissue.

Slide 23: Human Clock Gene Mutation – Familial Advanced-Phase Sleep Syndrome (FASPS)

Key Points:

• FASPS is a rare human sleep disorder where individuals fall asleep and wake up unusually early.

• It is linked to a mutation in the human Per2 gene.

• Genetic pedigree studies and body temperature/activity recordings show this inherited circadian disruption.

• Demonstrates that circadian clock genes directly influence human sleep behavior.

Explanation of Visuals:

• Pedigrees of affected families show dominant inheritance pattern.

• Actograms demonstrate earlier sleep and activity cycles in affected individuals.

• Diagram of the human Per2 gene locus identifies the mutation site.

Glossary:

• FASPS (Familial Advanced-Phase Sleep Syndrome): A genetic disorder causing early sleep-wake cycles.

• Per2: A key gene in the mammalian circadian clock.

Key Takeaway:
A mutation in the human Per2 gene causes FASPS, providing direct evidence that clock genes control human circadian behavior.

Slide 24: Why Do Clocks Exist?

Key Points:

• Poses a fundamental question:
  ➤ What is the evolutionary advantage of having a biological clock?

• Suggests that clocks must provide a survival or fitness benefit to organisms in a rhythmic world.

• It is some way of optimization, this is what circadian rhythms do. When is the right time to hunt? The right time to sleep? to eat?

  • Circadian rhythms exist to optimize our behaviour, our activism

Explanation of Visuals:

• Minimal text used to encourage discussion or reflection.

• Serves as a transition slide toward evolutionary studies of circadian rhythms.

Glossary:
(None specifically needed for this slide.)

Key Takeaway:
Biological clocks likely evolved to provide organisms with adaptive advantages in a rhythmic environment.

Slide 25: Circadian Rhythms and Evolution – Cyanobacteria

Key Points:

• In cyanobacteria, having an internal clock that matches the environment increases survival.

• Clock genes tuned to the external light-dark cycle lead to higher cell fitness and division.

• Mismatched clocks (e.g., 30h in a 24h environment) result in lower fitness.

• Likely reason: alignment optimizes energy processes like photosynthesis and nitrogen fixation.

Explanation of Visuals:

• Bar graphs show competitive growth advantage in bacteria with circadian clocks resonating with the light/dark cycle.

• Shows LD (light-dark) conditions vs. constant light.

Glossary:

• Resonating clock: A circadian rhythm that matches the external environment.

• Cyanobacteria: Photosynthetic microorganisms that have circadian rhythms.

Key Takeaway:
Organisms with circadian clocks tuned to the environment gain a survival advantage, demonstrating evolutionary pressure for clock alignment.

Slide 26: Circadian Clock Advantage in Plants

Key Points:

• In Arabidopsis (model plant), clock-aligned mutants grow better under natural light-dark cycles.

• Plants with internal clocks that match environmental cycles show:

  • Enhanced photosynthesis

  • Increased growth

  • Better competitive advantage

• Clock mutants that don’t align to day-night cycles show stunted growth.

Explanation of Visuals:

• Images of normal and mutant Arabidopsis plants under matched and mismatched light cycles.

• Plant with functional circadian rhythm grows more robustly under matching LD cycles.

Glossary:

• Arabidopsis thaliana: A small flowering plant used as a model in plant biology.

• Photoperiod: The daily length of light exposure.

Key Takeaway:
In plants, internal clocks aligned with the external environment promote optimal growth and survival—highlighting the evolutionary benefit of circadian regulation.

Slide 27: Circadian Rhythms of Social Synchronization in Honey Bees

Key Points:

• Honey bees exhibit socially synchronized circadian rhythms, important for colony organization and efficiency.

• Foragers show clear activity rhythms aligned with the time of day (e.g., visiting flowers at optimal times).

• This synchronization enhances pollination success and resource collection.

Explanation of Visuals:

• Photos show bees on flowers and in hives.

• Actograms display strong time-of-day specific activity patterns in forager bees.

Glossary:

• Social synchronization: Coordination of biological rhythms across individuals in a group.

• Forager bee: Worker bee responsible for collecting nectar and pollen outside the hive.

Key Takeaway:
Circadian rhythms help honey bees coordinate foraging activities with environmental cues, increasing colony survival and efficiency.

Slide 28: Resonating Clocks Enhance Fitness in Mice

Key Points:

• Mice with circadian periods matched to the environment (24h) show better survival compared to mismatched clocks (e.g., 20h or 28h).

• These experiments demonstrate that "resonance" between internal and external cycles increases lifespan.

• Mismatched rhythms lead to reduced fitness and longevity.

Explanation of Visuals:

• Actograms of mice with different circadian periods (short, normal, long).

• Graph shows survival curves, with 24h-clock mice living longer than mismatched ones.

Glossary:

• Resonating clock: Internal circadian rhythm that aligns with the 24-hour external environment.

• Fitness: Biological term for survival and reproductive success.

Key Takeaway:
In mammals, matching internal clocks to external cycles improves fitness and survival—highlighting the evolutionary pressure to align with day-night rhythms.

Slide 29: Circadian Clocks Anticipate Food Availability in Mammals

Key Points:

• Wild-type mice anticipate regular feeding times by becoming active before food is delivered.

• Per2 mutant mice (with disrupted clocks) fail to show anticipatory behavior.

• Indicates circadian clocks allow organisms to predict regular events like feeding time, even without external cues.

Explanation of Visuals:

• Actograms compare:

  • Wild-type mice: show anticipatory activity.

  • Per2 deficient mice: show delayed or absent anticipation.

• This illustrates how internal clocks prepare organisms for expected events.

Glossary:

• Food anticipation: A rise in activity before expected feeding time, regulated by the circadian clock.

Key Takeaway:
Mammalian circadian clocks enable animals to anticipate regular events like feeding, offering a behavioral advantage.

Slide 30: Clocks Anticipate Dawn and Dusk

Key Points:

• Circadian clocks prepare animals for predictable environmental changes, such as sunrise and sunset.

• This anticipation allows for timely physiological and behavioral adjustments (e.g., sleep, feeding, migration).

• Seen in both arctic and temperate animals, regardless of light intensity.

Explanation of Visuals:

• Arctic animal photos show adaptation to extreme light conditions.

• Actograms demonstrate daily activity rhythms aligned with light cycles, despite variability in natural light.

Glossary:

• Anticipation (circadian): Internal preparation for predictable daily changes before they occur.

Key Takeaway:
Biological clocks allow animals to anticipate environmental transitions like dawn and dusk, optimizing survival and activity.

Slide 31: Circadian Regulation of Physiology – How Do Clocks Regulate Behavior?

Key Points:

• The slide introduces a key question:
  ➤ How do circadian clocks influence daily behavior and bodily functions?

• Sets up the next slides to explore physiological processes governed by circadian timing.

Explanation of Visuals:

• Text-only slide prompting reflection or transition into physiological applications of circadian rhythms.

Glossary:
(None needed for this slide.)

Key Takeaway:
The circadian clock does more than control sleep—it orchestrates many physiological processes and behaviors.

Slide 32: Every Process Has an Optimal Circadian Period

Key Points:

• Circadian rhythms regulate many physiological systems, including:

  • Sleep-wake cycles

  • Heart rate and blood pressure

  • Body temperature

  • Hormone release

  • Kidney function and digestion

  • Mating and alertness

• There is an optimal time of day for each process to function most efficiently.

Explanation of Visuals:

• Central image of the brain clock with arrows showing how different organs and processes are regulated.

• Photos and diagrams represent rhythms in heart function, digestion, hormones, and animal behavior.

Glossary:

• Circadian physiology: The study of how internal clocks influence bodily functions.

Key Takeaway:
Circadian clocks coordinate the timing of numerous physiological processes to optimize function throughout the day.

Slide 33: The Central Clock Resides in the Suprachiasmatic Nucleus (SCN)

Key Points:

• The SCN in the hypothalamus is the master circadian clock in mammals.

  • SCN is the synchronizer, it synchronizes everything.

• It receives light input from the eyes and coordinates rhythms across the body.

• The SCN contains clock neurons that regulate various outputs to peripheral tissues.

Explanation of Visuals:

• Brain image showing the location of the SCN.

• Arrows indicate how light entrains the SCN, which in turn communicates with other brain areas and body systems.

Glossary:

• Suprachiasmatic nucleus (SCN): Brain region that acts as the central circadian pacemaker.

• Hypothalamus: Brain structure involved in many regulatory processes, including circadian rhythms.

Key Takeaway:
The SCN in the brain acts as the body’s central clock, receiving light input and synchronizing internal rhythms.

Slide 34: Master and Peripheral Clocks in the Body

Key Points:

• The SCN master clock synchronizes peripheral clocks in organs like the liver, heart, and kidney.

• The SCN uses:

  • Neural signals

  • Hormones (e.g., glucocorticoids)

  • Body temperature cycles

• These pathways ensure the whole body stays aligned with the external day-night cycle.

Explanation of Visuals:

• Diagram shows the SCN receiving light input from the eyes and sending outputs to peripheral clocks.

• Arrows represent synchronizing signals from the SCN to body tissues.

Glossary:

• Peripheral clocks: Circadian clocks found in organs and tissues outside the brain.

• Glucocorticoids: Steroid hormones that help synchronize peripheral clocks.

Key Takeaway:
The SCN master clock uses hormonal and neural pathways to coordinate peripheral clocks, keeping the body in sync with environmental time.

Slide 35: 2-deoxyglucose Uptake in Rat Brain Shows Circadian Activity

Key Points:

• Circadian rhythms influence brain activity at the metabolic level.

• Uptake of radioactively labeled glucose ([¹⁴C]-2-deoxyglucose) shows:

  • Higher SCN activity in the light phase (L-phase).

  • Lower SCN activity in the dark phase (D-phase).

• This experiment provides early evidence of daily metabolic rhythms in the brain.

Explanation of Visuals:

• Two brain scans show different intensities of glucose uptake in rats:

  • Left panel: high SCN activity in L-phase.

  • Right panel: reduced SCN activity in D-phase.

• Arrows point to the SCN region of the hypothalamus.

Glossary:

• [¹⁴C]-2-deoxyglucose: A radioactive glucose analog used to measure brain metabolic activity.

• SCN (Suprachiasmatic Nucleus): Master circadian clock in the brain.

Key Takeaway:
The SCN shows daily fluctuations in metabolic activity, demonstrating its role as a rhythm-generating brain region.

Slide 36: SCN Clock Neurons Are Diverse and Identified by Neuropeptides

Key Points:

• The SCN contains ~20,000 clock neurons, which are not all identical.

• It is not a homogenous gene-environement.

• Different groups express different neuropeptides, such as:

  • AVP (arginine vasopressin): dorsal SCN.

  • VIP (vasoactive intestinal peptide): ventral SCN.

    • The VIP neurons are the first ones that receive the sunlight inputs

    • They sit at the top.

• These subpopulations may have specialized roles in coordinating rhythms.

Explanation of Visuals:

• Fluorescent images show localization of AVP and VIP within the SCN.

• Schematic of SCN illustrates distinct neuropeptide zones (dorsal vs. ventral).

Glossary:

• Neuropeptide: A small protein-like molecule used by neurons to communicate.

• AVP/VIP: Specific neuropeptides involved in circadian signaling.

Key Takeaway:
The SCN is composed of multiple neuron types, each marked by different neuropeptides, supporting diverse roles in timekeeping.

Slide 37: SCN Neurons Oscillate in Slightly Different Phases

Key Points:

• Individual SCN neurons do not peak in activity at exactly the same time.

• There is phase variability across the neuronal population.

• This variability allows for robust and flexible circadian output.

Explanation of Visuals:

• Fluorescent image shows many SCN neurons glowing at once, but with subtle timing differences.

• Circular diagram shows phase dispersion among dorsal and ventral SCN neurons.

Glossary:

• Oscillation phase: The timing of the peak of a rhythmic activity (e.g., gene expression or firing).

Key Takeaway:
SCN neurons are synchronized but exhibit slight phase differences, enabling a more nuanced and stable circadian rhythm.

Slide 38: SCN Clock Neurons Respond Differently to Long and Short Days

Key Points:

• The SCN adapts to seasonal changes in day length by altering the timing of neuron activity.

• In long days, neuron peak times are broader and more dispersed.

• In short days, peaks are tighter and more synchronized.

• This adjustment allows the body to track seasonal time.

Explanation of Visuals:

• Heatmaps show peak activity times of SCN neurons under long and short days.

• Circular schematic shows shifts in neuronal phase relationships depending on photoperiod.

Glossary:

• Photoperiod: The length of the light period in a 24-hour day.

• CT (Circadian Time): A time standard based on internal clock phase, independent of external cues.

Key Takeaway:
The SCN adjusts internal timing of its neurons to reflect changes in day length, supporting seasonal adaptation in physiology and behavior.

Slide 39: The Clock Must Enable Seasonal Adaptation

Key Points:

• Circadian clocks must also adjust to seasonal changes in day length (photoperiod).

• Animals use melatonin secretion patterns to detect day length:

  • Longer nights → longer melatonin release.

  • Shorter nights → shorter melatonin release.

• These changes in melatonin duration inform the body of the season and help regulate reproduction, metabolism, and behavior.

Explanation of Visuals:

• Diagrams show melatonin release profiles:

  • Winter (long night): Extended melatonin signal.

  • Summer (short night): Shorter melatonin signal.

• The SCN adapts internal timing to adjust hormonal signaling accordingly.

Glossary:

• Melatonin: Hormone produced during darkness; communicates day length to the body.

• Photoperiodism: Biological response to changes in day or night length.

Key Takeaway:
The circadian clock helps organisms interpret and adapt to seasonal changes through variations in melatonin signaling.

Slide 40: Targets of the SCN

Key Points:

• The SCN projects to multiple brain regions to coordinate and synchronizes:

  • Hormone release

  • Body temperature

  • Sleep-wake cycles

  • Feeding behavior

• These outputs enable synchronization of daily and seasonal rhythms across the entire organism.

Explanation of Visuals:

• Diagram of the brain shows SCN and its downstream targets.

• Graphs illustrate rhythmic outputs such as hormone secretion and locomotor activity.

Glossary:

• SCN projections: Neural connections from the SCN to other brain areas.

Key Takeaway:
The SCN acts as a central hub, distributing circadian timing information to key physiological systems via neural pathways.

Slide 41: The SCN Is Required for Rhythmic Behavior

Key Points:

• Ablation (removal) of the SCN in rats eliminates circadian activity rhythms.

• Without the SCN, rats display arrhythmic behavior even in constant conditions.

• This confirms the SCN is essential for generating rhythmic patterns of activity.

Explanation of Visuals:

• Actograms compare:

  • Pre-SCN ablation: clear, daily activity cycles.

  • Post-SCN ablation: disorganized, arrhythmic activity.

Glossary:

• SCN ablation: Surgical removal or lesioning of the suprachiasmatic nucleus.

• Arrhythmic: Lacking regular circadian patterns.

Key Takeaway:
The SCN is necessary for the expression of circadian rhythms in behavior—without it, rhythms disappear.

Slide 42: Transplantation Proves the SCN Controls Rhythmicity

Key Points:

• SCN transplantation restores rhythmicity in SCN-lesioned animals.

• The donor SCN's rhythm determines the recipient's behavior:

  • E.g., a 20-hour rhythm from the donor leads to a 20-hour cycle in the host.

• This proves the SCN is not just necessary but sufficient to impose a circadian rhythm.

Explanation of Visuals:

• Graphs show restored rhythmic behavior in SCN-transplanted animals. Rat with knockout SCN received from the donor a SCN and rats got rhythms same as from the donor rat.

• One group receives a wild-type SCN, the other receives a short-period mutant SCN.

Glossary:

• SCN transplantation: Implanting a donor SCN into a host animal.

• Sufficient: Capable on its own of driving the behavior.

Key Takeaway:
The SCN alone can control circadian rhythms, proving it acts as the master clock that drives behavioral timing.

Slide 43: SCN Neurons Show Daytime Activity and Low Nighttime Activity

Key Points:

• Individual SCN neurons are more electrically active during the day and less active at night.

• This daily rhythm in neuronal firing is an intrinsic property of the SCN.

• Electrical recordings show a clear 24-hour rhythm in firing rates.

Explanation of Visuals:

• Top graph shows neuronal firing rate peaking during the light phase.

• Bottom panel shows raster plots of spontaneous firing activity over time in multiple neurons.

Glossary:

• Spontaneous activity: Electrical activity that occurs without external stimulation.

• Firing rate: Frequency at which neurons generate action potentials.

Key Takeaway:
SCN neurons display intrinsic day-night rhythms in electrical activity, a fundamental feature of circadian timekeeping.

Slide 44: The Big Surprise – Each Cell Has Its Own Clock

Key Points:

• When cultured individually, SCN neurons continue to oscillate independently.

• Each neuron has its own circadian rhythm, even outside the brain.

• This shows the circadian clock is cell-autonomous.

Explanation of Visuals:

• Traces of gene expression or electrical activity in individual neurons.

• Diagrams illustrate rhythmic behavior in isolated cells.

Glossary:

• Cell-autonomous: A process that occurs within a cell independently of other cells.

Key Takeaway:
Each SCN neuron contains its own molecular clock, capable of maintaining circadian rhythms without external signals.

Slide 45: Neuronal Firing Is Not Required to Keep Time

Key Points:

• Blocking neuronal activity (firing) does not stop the molecular clock.

• SCN cells still show rhythmic gene expression even when action potentials are blocked.

• However, neurons still have individual clock periods, emphasizing independent oscillators.

Explanation of Visuals:

• Graphs show that blocking sodium channels (which stops firing) does not disrupt the rhythmic clock gene expression.

• Red text highlights the independence of the molecular clock from electrical activity.

Glossary:

• TTX (Tetrodotoxin): A chemical that blocks neuronal firing.

• Molecular clock: The set of genes and proteins that generate circadian rhythms inside cells.

Key Takeaway:
The molecular circadian clock continues to tick even when neurons are electrically silent—firing is not required to keep time.

Slide 46: VIP Mediates Synchrony in the SCN

Key Points:

• Vasoactive intestinal polypeptide (VIP) is crucial for synchronizing SCN neurons.

• Without VIP, individual neurons keep time but lose coordination with each other.

• VIP enables the SCN to function as a coherent clock network.

Explanation of Visuals:

• Firing activity in SCN cultures shows desynchronized rhythms without VIP signaling.

• With VIP, neuron rhythms are tightly aligned.

Glossary:

• VIP (Vasoactive Intestinal Peptide): A signaling molecule important for intercellular communication in the SCN.

• Synchrony: Alignment of rhythms across multiple oscillators.

Key Takeaway:
While each SCN neuron has its own clock, VIP is essential for keeping all the clocks in sync, ensuring unified circadian output.

Slide 47: The SCN Is Required for Normal Sleep Timing in Mice

Key Points:

• The suprachiasmatic nucleus (SCN) is essential for maintaining normal timing of sleep-wake cycles in mice.

• Mice with an intact SCN sleep at regular, expected times.

• Disruption or deletion of clock genes in the SCN causes disorganized or mistimed sleep.

Explanation of Visuals:

• Line graphs show the timing of sleep and wake phases in wild-type vs. SCN-deficient mice.

• Disrupted SCN leads to a blunted or erratic sleep rhythm compared to a robust rhythm in normal mice.

Glossary:

• Sleep timing: The regular scheduling of sleep within a 24-hour period.

• SCN (Suprachiasmatic Nucleus): The central circadian clock in the brain.

Key Takeaway:
The SCN is essential for organizing when sleep occurs, ensuring it aligns with the external day-night cycle.

Slide 48: How Do Clock Neurons Regulate Daily Sleep-Wake Cycles?

Key Points:

• SCN activity drives the daily transition between sleep and wakefulness.

• Mice with SCN disruption lose this rhythmic control, especially during light-dark transitions.

• Neuronal signals from specific SCN subregions (VIP and AVP zones) coordinate these transitions.

• Mice shown to have subregions in SCN which were managing a “siesta-time” meaning shutting done the activities little before and it is regulated by few VIP neurons.

Explanation of Visuals:

• Actograms compare control vs. SCN-ablated mice.

• Diagram shows subregions of the SCN involved in coordinating behavioral rhythms.

• Bar graph highlights loss of sleep-wake transitions in mutant animals.

Glossary:

• Sleep-wake cycle: The circadian regulation of alternating sleep and wake states.

Key Takeaway:
The SCN sends coordinated signals that trigger daily shifts between sleep and wakefulness, organizing rest-activity behavior.

Slide 49: The SCN Is Active When Mice Sleep

Key Points:

• In nocturnal animals like mice, the SCN is most active during the day (when mice sleep).

• SCN activity is inversely related to behavioral activity—it peaks during the rest phase.

• This suggests that SCN may actively suppress wakefulness during the rest phase.

Explanation of Visuals:

• Graphs show high SCN activity (e.g., neuronal firing) during the light period.

• Bar chart correlates SCN activity with sleep timing in mice.

Glossary:

• Nocturnal: Active at night; sleeps during the day.

Key Takeaway:
In mice, the SCN is highly active when they are asleep, suggesting its role in promoting or maintaining rest during the inactive phase.

Slide 50: How Does the SCN Regulate Activity at Night?

Key Points:

• During the active night phase, the SCN becomes less active, which allows arousal and movement.

• SCN may indirectly regulate nighttime activity by reducing inhibitory signals to arousal centers.

• This timing of SCN inhibition is essential for normal nocturnal behavior in mice.

Explanation of Visuals:

• SCN activity drops at night in mice, coinciding with increased physical activity.

• Arrows indicate SCN’s influence on downstream brain areas that promote wakefulness or sleep.

Glossary:

• Arousal centers: Brain regions that promote wakefulness and alertness.

Key Takeaway:
The SCN reduces its output at night in nocturnal animals, releasing suppression of activity and enabling nighttime behaviors.

Slide 51: A Normal Siesta Requires VIP+ SCN Neurons

Key Points:

• A midday siesta (rest period) in mice is regulated by VIP-expressing neurons in the SCN.

• When VIP+ SCN neurons are disrupted, the typical dip in activity during midday disappears.

• This suggests that VIP+ neurons are essential for initiating or maintaining rest in the early afternoon.

Explanation of Visuals:

• Activity graphs show:

  • Normal mice have a drop in activity during midday (siesta).

  • Mice lacking functional VIP+ SCN neurons lack this dip, showing continuous activity.

• Red arrow and “VIP” label emphasize the role of VIP neurons in this rest period.

Glossary:

• VIP+ neurons: Neurons in the SCN that produce vasoactive intestinal peptide, important for circadian signaling.

• Siesta: A rest or sleep phase typically occurring in the early afternoon.

Key Takeaway:
VIP+ SCN neurons are required for normal midday rest, helping coordinate the timing of a siesta-like pause in activity.

Slide 52: VIP+ SCN Neurons Promote Siesta Quiescence and Sleep

Key Points:

• Activation of VIP+ SCN neurons increases quiescence (restfulness) and sleep during the siesta period.

• The more VIP neurons are activated, the greater the sleep-promoting effect.

• These neurons appear to fine-tune the depth and duration of the siesta.

Explanation of Visuals:

• Activity and sleep traces show enhanced rest behavior in mice when VIP+ neurons are active.

• Graphs compare baseline vs. stimulated conditions.

• The effect is absent or reduced in mice lacking VIP neuron function.

Glossary:

• Quiescence: A state of inactivity or minimal movement; often associated with rest or sleep.

• Stimulation (optogenetic): Experimental technique to activate neurons with light.

Key Takeaway:
VIP+ SCN neurons actively promote siesta sleep and rest, highlighting their key role in regulating daily rest phases.

Slide 54: Circadian Locomotor Activity – The Wheel Running Assay

Key Points:

• The wheel running assay is a standard method to study circadian rhythms in rodents.

• Mice housed with a running wheel show strong rhythmic activity, especially during the dark phase.

• The assay allows for quantification of circadian behavior in response to light-dark cycles or constant conditions.

Explanation of Visuals:

• Left: Computer and wheel setup for tracking activity.

• Right: Actogram showing regular activity during dark periods (shaded) and rest during light.

• Data indicates clear circadian patterning of locomotion.

Glossary:

• Actogram: A graphical representation of activity across time.

• Locomotor activity: Movement behavior, such as running on a wheel.

Key Takeaway:
The wheel running assay is a powerful tool to measure circadian rhythms in animal behavior, especially locomotion.

Slide 55: Circadian Research in Humans

Key Points:

• Human circadian research explores biological timing and its effects on behavior, sleep, and physiology.

• The metaphorical image (human in a hamster wheel) humorously reflects our own daily cycles and routines.

• Research in humans uses non-invasive methods to track sleep and activity.

Explanation of Visuals:

• The image shows a man in a giant hamster wheel, symbolizing daily repetition and circadian-driven behavior.

Glossary:

• (No specific glossary terms needed for this metaphorical slide.)

Key Takeaway:
Humans, like animals, are influenced by internal biological clocks that regulate daily behaviors.

Slide 56: Jürgen Aschoff and Human Bunker Experiments

Key Points:

• Jürgen Aschoff was a pioneer in studying human circadian rhythms.

• In bunker experiments, volunteers lived without external time cues (no light/dark cycle).

• Even in isolation, people maintained near-24-hour rhythms, proving the existence of an internal clock.

Explanation of Visuals:

• Left: Photo of Jürgen Aschoff.

• Right: Graphs from bunker studies showing patterns in body temperature, sleep-wake cycles, and activity.

• Rhythms persisted but could free-run slightly outside 24 hours.

Glossary:

• Free-running rhythm: A circadian rhythm that is not synchronized by external cues.

Key Takeaway:
Aschoff’s studies confirmed that humans have intrinsic circadian rhythms that persist even in isolation.

1. How Do Time Zone Shifts Affect Our Circadian Rhythm?

When flying from New York to Los Angeles, you're traveling west, gaining 3 hours — i.e., your day becomes longer.

When flying from Los Angeles to New York, you're traveling east, losing 3 hours — your day becomes shorter.

So it’s like this:

✅ 2. Which Is Harder: Gaining or Losing Hours?

📌 It’s scientifically more difficult for most people to adjust when losing hours (flying east).

🧠 Why? (Scientific Reasons):

🔬 A. Human Circadian Rhythm Is Slightly Longer Than 24 Hours

• Jürgen Aschoff’s bunker experiments (like in your Slide 56 summary) showed that without time cues, our internal clock tends to run a little longer than 24 hours—around 24.2 to 24.5 hours in many people.

• ➤ This means it’s easier to stay awake longer (delay sleep) than to fall asleep earlier (advance sleep).

🔬 B. Phase Delay vs. Phase Advance

• Westward travel requires a phase delay (go to bed later):

  • This matches our natural tendency to extend the day → easier.

• Eastward travel requires a phase advance (go to bed earlier):

  • This goes against our biology → harder to fall asleep early and wake up early.

🔬 C. Melatonin and Light Exposure

• Your body uses light and melatonin to regulate your clock.

• After eastward flights, your internal clock still thinks it's earlier than it is, so you might:

  • Feel wide awake at night.

  • Struggle with early morning obligations.

• Adjusting melatonin production and getting proper light exposure takes several days.

Slide 57: Modern Circadian Research in Humans

Key Points:

• Today’s circadian research uses wearable devices and non-invasive monitoring to assess rhythms.

• Tools like the Octagonal Basic Motionlogger track movement and sleep patterns.

• This research helps in:

  • Understanding sleep disorders

  • Optimizing shift work and jet lag treatment

  • Assessing chronobiology in infants and the elderly

Explanation of Visuals:

• Photos of babies, shift workers, and the elderly highlight diverse research subjects.

• Device shown on wrist demonstrates a modern actigraphy tool.

Glossary:

• Actigraphy: A method of monitoring human rest/activity cycles using wearable sensors.

Key Takeaway:
Modern technology allows researchers to study circadian rhythms in real-life human settings using wearable monitoring tools.

• The activity pattern of the infant is fragmented and irregular across day and night.

• Over time, rhythms start to emerge, showing progressive consolidation of sleep-wake cycles.

Explanation of Visuals:

• Actogram of an infant’s activity over the first 90 days.

• Shows frequent awakenings and no clear differentiation between day and night early on.

• Gradual pattern formation is seen by the end of the third month.

Glossary:

• Actogram: A visual representation of activity across time.

• Sleep-wake consolidation: Development of a clear distinction between nighttime sleep and daytime wakefulness.

Key Takeaway:
Human infants are born without strong circadian rhythms; these patterns gradually develop in the first few months of life.

Slide 58: Circadian Rhythms in a Human Infant (First 3 Months)

Key Points:

• In early infancy, circadian rhythms are not yet fully established.

• The activity pattern of the infant is fragmented and irregular across day and night.

• Over time, rhythms start to emerge, showing progressive consolidation of sleep-wake cycles.

Explanation of Visuals:

• Actogram of an infant’s activity over the first 90 days.

• Shows frequent awakenings and no clear differentiation between day and night early on.

• Gradual pattern formation is seen by the end of the third month.

Glossary:

• Actogram: A visual representation of activity across time.

• Sleep-wake consolidation: Development of a clear distinction between nighttime sleep and daytime wakefulness.

Key Takeaway:
Human infants are born without strong circadian rhythms; these patterns gradually develop in the first few months of life.

Slide 59: Circadian Rhythms in the Mother of the Infant

Key Points:

• The mother’s actogram shows well-defined and consistent sleep-wake cycles.

• There is evidence of disturbance and fragmentation, likely due to caring for the infant.

• Despite interruptions, the mother maintains a clear day-night rhythm.

Explanation of Visuals:

• Actogram of the same period (first 90 days) from the infant’s mother.

• Shows overall rhythmic activity but with some night awakenings or fragmented rest periods.

Glossary:

• Fragmented sleep: Sleep that is broken into multiple episodes, often due to external interruptions.

Key Takeaway:
Mothers retain a circadian rhythm but experience disrupted sleep when caring for a newborn with undeveloped rhythms.

Slide 60: Chronotypes and Circadian Period Lengths

Key Points:

• Human sleep-wake preferences (chronotypes) relate to the length of their internal circadian period:

  • Shorter period → phase advance → "Early Birds" / Advanced Sleep Phase Syndrome.

  • Longer period → phase delay → "Night Owls" / Delayed Sleep Phase Syndrome.

• Chronotypes exist along a spectrum from morning types to evening types.

• Everything longer than 24.2 hours of spectrum are conisdered as delayed phases

Explanation of Visuals:

• A gradient diagram shows the relationship between period length and preferred sleep timing.

• Clinical sleep syndromes like ASPS and DSPS are positioned at the extremes.

  • Advanced Sleep Disorder Syndrom / Delayed Sleep Disorder Syndrom

Glossary:

• Chronotype: An individual’s natural preference for sleep and activity timing.

• Phase advance/delay: Shift of the sleep-wake cycle earlier or later, respectively.

Key Takeaway:
People differ in circadian timing; those with shorter or longer internal rhythms show advanced or delayed sleep preferences, respectively.

Slide 61: Chronotype Metaphor – Watch Types

Key Points:

• The slide uses an analogy with watches to explain chronotypes:

  • A fast-running watch mimics a short circadian period, leading to early rising.

  • A slow-running watch mimics a long circadian period, leading to late sleeping.

• These biological "watch speeds" influence individual sleep habits and social timing.

Explanation of Visuals:

• Two watches and their metaphoric links to sleep timing:

  • Fast watch = Early appointments.

  • Slow watch = Late appointments.

• Clear visual link between internal timing and real-life behavior.

Glossary:

• Internal clock speed: The length of one circadian cycle (usually close to 24 hours, but slightly varies by person).

Key Takeaway:
Just like watches can run fast or slow, people’s biological clocks differ in speed, influencing whether they are early risers or night owls.

Slide 62: Period Length Correlates with Chronotype

Key Points:

• A study shows a correlation between period length and HO score (a questionnaire measuring chronotype).

• Individuals with shorter circadian periods tend to be morning types (early birds).

• Those with longer periods tend to be evening types (night owls).

Explanation of Visuals:

• Scatterplot graph:

  • Y-axis: Period length (in hours).

  • X-axis: HO score (higher = more evening-oriented).

• Data points suggest a positive trend: longer periods align with later chronotypes.

Glossary:

• Period length: Duration of a full circadian cycle (typically near 24 hours).

• HO score: Horne–Östberg score, used to assess morningness/eveningness.

Key Takeaway:
Your internal clock’s period length plays a role in determining whether you are a morning or evening person.

What the Graph Shows:

This scatterplot illustrates the relationship between circadian period length and chronotype, as measured by the HO (Horne–Östberg) score.

• The y-axis shows the period length in hours (the duration of a person’s internal circadian cycle).

• The x-axis shows the HO score, which quantifies chronotype. A low score indicates a morning type (early bird), while a high score indicates an evening type (night owl).

• Each magenta dot represents an individual. The vertical error bars show the variation or uncertainty in measuring their circadian period.

Interpretation:

• Individuals with lower HO scores (morning types) tend to have shorter period lengths, closer to or below 24.5 hours.

• Individuals with higher HO scores (evening types) tend to have longer period lengths, often close to or above 25 hours.

• This suggests a trend: the longer your internal circadian cycle, the more likely you are to prefer evening activity.

Glossary:

• Period length: The time it takes for a person’s internal biological clock to complete one full cycle, typically close to 24 hours.

• HO score (Horne–Östberg score): A questionnaire-based measure used to classify people as morning or evening types.

Key Takeaway:

An individual’s natural circadian period is associated with their chronotype. Shorter periods are linked to morning preference, while longer periods are associated with evening preference.

Slide 63: Clocks and the Environment

Key Points:

• The circadian system affects real-world events, such as:

  • Traffic accidents, which spike in the early morning.

  • Health risks from night shift work or poor sleep alignment.

• Disruption between internal clocks and environmental time (circadian misalignment) can have serious consequences.

Explanation of Visuals:

• Left: Graph showing increased traffic accidents around 8 a.m.

• Middle: Pie chart shows distribution of shift workers.

• Bottom: Image of Chernobyl nuclear disaster sign, referencing its timing during a night shift.

Glossary:

• Circadian misalignment: Mismatch between the body’s internal clock and the external environment.

• Shift work disorder: Health problems caused by working outside normal sleep hours.

Key Takeaway:
Disruptions to our circadian alignment, especially in the context of shift work or sleep deprivation, can have significant safety and health implications.

Slide 64: How Does the Clock Stay Entrained with the Environment?

Key Points:

• The internal clock stays synchronized to the environment through light exposure.

• Light acts as a zeitgeber (time-giver), shifting the clock’s phase to align with the day-night cycle.

• This ensures the correct activation of clock genes at the right time of day.

Explanation of Visuals:

• Text-only slide with emphasis on the role of light as a primary entrainment cue.

Glossary:

• Entrainment: The process of syncing an internal clock to an external time cue.

• Phase-shift: Adjustment of the timing of circadian rhythms in response to a stimulus like light.

• Zeitgeber: An external cue (like light) that helps reset the biological clock.

Key Takeaway:
Light keeps the circadian clock aligned with the environment by resetting its phase and driving daily rhythms.

Slide 65: Clocks Are Everywhere

Key Points:

• Circadian clocks exist not only in the brain (SCN) but also in many peripheral organs:

  • Liver, lungs, intestines, kidneys, etc.

• These clocks help regulate tissue-specific functions, and are entrained by the master SCN clock and other cues like feeding.

Explanation of Visuals:

• Diagram of the body highlighting multiple tissues with their own clocks.

• Arrows indicate communication between central and peripheral clocks.

Glossary:

• Peripheral clocks: Circadian clocks located in tissues outside the brain.

• SCN (Suprachiasmatic Nucleus): The master clock that coordinates rhythms across the body.

Key Takeaway:
The circadian system is a network—while the SCN is the master clock, virtually every organ in the body has its own local timekeeper.

Slide 66: Model of the Mammalian Circadian Oscillator

Key Points:

• The circadian clock operates via a transcription-translation feedback loop (TTFL) involving clock genes.

• BMAL1 and CLOCK proteins activate transcription of Per and Cry genes.

• PER and CRY proteins then inhibit their own transcription, forming a feedback loop.

• Light signals via CREB, cAMP, and Ca²⁺ pathways regulate gene expression in response to environmental cues.

• Phase shifts can be triggered by light, altering the expression of clock genes and resetting the rhythm.

Explanation of Visuals:

• Diagram shows the molecular components of the circadian clock in mammals.

• Emphasizes light-driven regulation of Per1/Per2 genes via signaling cascades.

Glossary:

• TTFL (Transcription-Translation Feedback Loop): Core mechanism of circadian clocks involving cyclical gene expression.

• Phase shift: Change in the timing of the circadian clock.

Key Takeaway:
The mammalian circadian oscillator is built on a self-regulating gene loop that responds to environmental light to shift its phase.

Slide 67: Light Resets the Clock – Phase Response by Species

Key Points:

• Light acts as a zeitgeber to reset the circadian clock across species.

• The timing of light exposure determines whether it causes a phase delay (shift later) or phase advance (shift earlier).

• This mechanism is conserved in species from fish to mice and humans.

Explanation of Visuals:

• Illustrations compare how light pulses at different times result in opposite phase shifts depending on the timing.

• Arrows show whether rhythms are advanced or delayed.

Glossary:

• Phase advance: Moving the circadian rhythm earlier.

• Phase delay: Moving the rhythm later.

• Zeitgeber: External cue, like light, that synchronizes internal clocks.

Key Takeaway:
The effect of light on circadian rhythms depends on the time of exposure, and this principle applies across many organisms.

Slide 68: Light Induces Per1 and Per2 Gene Expression in the SCN

Key Points:

• Light exposure at night causes increased expression of Per1 and Per2 genes in the SCN.

• These genes are part of the core clock mechanism and are involved in resetting the rhythm.

• The timing and intensity of light determine the magnitude of Per gene induction.

• Light causes High Expression of Per1 (see in bottom barplot)

Explanation of Visuals:

• Top: Fluorescence images of SCN slices show Per1/Per2 expression after light.

• Bottom: Bar graphs quantify gene expression levels at different time points post light exposure.

Glossary:

• SCN (Suprachiasmatic Nucleus): Brain region coordinating circadian rhythms.

• Per1/Per2: Clock genes involved in feedback regulation of the circadian clock.

Key Takeaway:
Light directly induces Per gene expression in the SCN, providing a molecular mechanism for phase resetting.

Slide 69: Human Phase Response Curve (PRC) to Light

Key Points:

• The human phase response curve shows how light affects the clock differently depending on the time of day.

• Light in the early night causes a phase delay.

• Light in the early morning leads to a phase advance.

• Light around mid-day has minimal effect.

Explanation of Visuals:

• Graph plots the magnitude and direction of phase shifts after a single light pulse at various circadian times.

• X-axis: Time of light exposure; Y-axis: Hours shifted.

Glossary:

• Phase response curve (PRC): Graph showing how timing of a stimulus (e.g. light) alters circadian phase.

Key Takeaway:
Your biological clock responds to light differently depending on when you see it—this is critical for sleep timing and chronotherapy.

Slide 70: Phase Advance vs. Phase Delay by Light

Key Points:

• Light at different circadian phases can cause either:

  • Phase delay (e.g., late evening)

  • Phase advance (e.g., early morning)

• These shifts are linked to light-induced changes in gene expression within the clock.

Explanation of Visuals:

• Diagram shows clock gene oscillations and how light can reset these curves either forward or backward.

• Lower panel illustrates molecular pathway by which light leads to changes in the TTFL.

Glossary:

• Oscillation phase: The position within a rhythmic cycle.

• TTFL (Transcription-Translation Feedback Loop): Genetic feedback loop at the heart of the circadian clock.

Key Takeaway:
Light fine-tunes circadian phase by advancing or delaying the internal clock, depending on when it is perceived.

Slide 71: Circadian Photoreception Does Not Depend on Rods and Cones

Key Points:

• Surprisingly, classical photoreceptors (rods and cones) are not required for circadian light detection.

• Circadian behaviors like activity rhythms and gene expression shifts in response to light persist even in mice lacking rods and cones.

• This indicates the existence of non-visual photoreceptors in the eye.

Explanation of Visuals:

• Graphs show responses (e.g. Per gene induction) in genetically modified mice missing rods/cones.

• Despite the loss of image-forming vision, circadian photoentrainment remains intact.

• Meaning, that also if people/an organism is blind and still the eyes are there, the receptors that take on the sunlight and still transmit it to the brain. The problem would be if there are no eyes anymore.

• Eyes are therefore also important even if people are blind.

Glossary:

• Photoreception: Light detection by the eye.

• Rods/Cones: Classical photoreceptor cells used for vision.

• Photoentrainment: Synchronization of the internal clock to light.

Key Takeaway:
Circadian light sensing does not rely on visual photoreceptors but is still an ocular function.

Slide 72: Discovery of ipRGCs – Non-Visual Photoreceptors in the Eye

Key Points:

• Specialized retinal cells called ipRGCs (intrinsically photosensitive Retinal Ganglion Cells) mediate circadian light sensing.

• These cells contain melanopsin, a light-sensitive protein, and form the non-image-forming light pathway.

• ipRGCs send signals to the SCN and other brain areas to regulate circadian behavior and physiology.

Explanation of Visuals:

• Left: Graph shows melanopsin’s light sensitivity curve, different from rods/cones.

• Right: Retinal histology shows location of ipRGCs.

Glossary:

• ipRGCs: Retinal ganglion cells that are intrinsically sensitive to light and project to the brain → even when blind people, with this rhythmus are still present.

• Melanopsin: A photopigment in ipRGCs responsible for circadian photoreception.

Key Takeaway:
Non-visual retinal cells (ipRGCs) detect light for circadian regulation, independent of classical visual systems.

Slide 73: Melanopsin is Required for Full Light Response in Circadian System

Key Points:

• Mice without melanopsin show impaired phase shifting in response to light.

• Combined mutations in rods/cones and melanopsin abolish light response altogether.

• This confirms that melanopsin is essential for circadian photoentrainment, especially when rods and cones are absent.

Explanation of Visuals:

• Actograms show light-induced phase shifts in different mouse genotypes.

• Mice lacking melanopsin show reduced or absent adjustments in behavior following light exposure.

Glossary:

• Phase shift: Change in timing of circadian rhythms.

• Melanopsin knockout: Genetically modified animals lacking melanopsin.

Key Takeaway:
Melanopsin in ipRGCs is essential for mediating circadian light responses, especially when other photoreceptors are missing.

Slide 74: Light Signals Reach SCN via the Retinohypothalamic Tract (RHT)

Key Points:

• Light information from ipRGCs travels through the retinohypothalamic tract (RHT) to reach the SCN.

• Only a subset of SCN neurons receives these light-dependent signals.

• These signals are crucial for resetting the circadian clock.

Explanation of Visuals:

• Left: Actograms showing light responsiveness in SCN-connected mice.

• Right: Schematic showing the RHT’s projection to the SCN and specific cell populations involved.

Glossary:

• RHT (Retinohypothalamic Tract): Neural pathway connecting the retina to the SCN.

• SCN: Suprachiasmatic Nucleus, the brain’s master clock.

Key Takeaway:
Light entrains the circadian system via a specialized neural pathway (RHT) from the retina to the SCN.

Slide 75: Light Input Signaling to the SCN (This slight eas mentioned to be important)

Key Points:

• Light activates ipRGCs, which send glutamatergic signals via the RHT to SCN neurons.

• This triggers calcium influx and activates CREB, leading to the transcription of clock genes (e.g., Per1/2).

• This is the molecular mechanism of light-induced phase resetting.

Explanation of Visuals:

• Diagram of the light signal transduction pathway:

  • Light → ipRGC → RHT → SCN neuron → glutamate → Ca²⁺ → CREB → gene transcription.

Glossary:

• CREB: A transcription factor activated by calcium signaling.

• Glutamate: Major excitatory neurotransmitter in the brain.

Key Takeaway:
Light entrains the SCN through a defined molecular pathway involving glutamate, calcium, and clock gene activation.

📍Step-by-Step Breakdown:

1. Light Detection

• Sunlight reaches the eye, and light-sensitive cells in the retina detect this light.

2. RHT = Retinohypothalamic Tract

• A neural pathway connecting the retina to the SCN (suprachiasmatic nucleus) in the hypothalamus.

• RHT transmits the light signal using the neurotransmitter glutamate (GLU).

3. SCN Neuron Activation

• Glutamate (GLU) binds to receptors on the SCN neuron.

• This triggers an influx of calcium ions (Ca²⁺) into the cell.

4. Calcium-Activated Signaling Pathway

• The rise in Ca²⁺ leads to activation (phosphorylation) of various kinases:

🧠 These kinases trigger phosphorylation of CREB.

5. CREB = cAMP Response Element-Binding Protein

• Phosphorylated CREB (p-CREB) binds to CRE (cAMP response element) sites in the DNA.

• This activates transcription of clock genes, like Per1 and Per2.

6. E-box Activation

• Clock genes also contain E-box sequences, which are DNA elements regulated by CLOCK/BMAL1.

• Light-driven CREB activation can influence expression through CRE and possibly E-boxes.🧬 Summary of Terms & Abbreviations:

Slide 76: Molecular Pathways Regulating Clock Gene Expression (This slide also as important to understand mentioned)

Key Points:

• Light stimulation triggers signaling cascades via G proteins and cAMP.

• These pathways activate MAPK (Mitogen-Activated Protein Kinase) and CREB.

• Activated CREB stimulates transcription of Per1/2, resetting the clock’s rhythm.

• The pathway shows how external light signals are translated into molecular changes within clock neurons.

Explanation of Visuals:

• Diagram showing the signal transduction pathway from G protein to clock gene transcription.

• Highlights key intermediates like MAPK and CREB.

Glossary:

• cAMP: A signaling molecule activated by G-proteins.

• CREB: Transcription factor regulating Per gene expression.

• Per1/2: Core clock genes responsible for feedback regulation in circadian rhythms.

• G proteins (short for guanine nucleotide-binding proteins) are molecular switches that help transmit signals from receptors on the cell surface to the inside of the cell.

Key Takeaway:
Light resets the circadian clock through a defined molecular cascade involving CREB activation and Per gene transcription.

Slide 77: Circadian Dysregulation of Physiology – What Happens When the Clock Fails?

Key Points:

• Disruption of circadian rhythms can lead to physiological and mental health issues.

• This slide introduces the concept of circadian dysfunction, preparing for examples in sleep and mood disorders.

Explanation of Visuals:

• Title slide prompting the discussion of clock-related health consequences.

• Poses the question: "What happens when the clock goes bad?"

Key Takeaway:
Circadian rhythm disruptions are linked to a wide range of physiological and psychological disorders.

Slide 78: Familial Advanced Sleep Phase Syndrome (FASPS)

Key Points:

• FASPS is a genetic disorder caused by mutations in the Per2 gene.

• Affected individuals have significantly advanced sleep-wake cycles (they fall asleep and wake very early).

• A single point mutation in Per2 was identified in several families.

Explanation of Visuals:

• Left: Pedigrees of affected families.

• Right: Actograms showing body temperature rhythms.

• Bottom: Map of human Per2 gene and mutation location.

Glossary:

• FASPS: A hereditary sleep disorder with early sleep timing.

• Point mutation: A single base change in the DNA sequence.

Key Takeaway:
A mutation in the Per2 gene causes familial advanced sleep phase syndrome, shifting sleep timing significantly earlier.

Slide 79: Neurological Disorders with Disrupted Sleep

Key Points:

• Sleep disturbances are common in neuropsychiatric conditions, such as:

  • Schizophrenia

  • Major depressive disorder

  • Bipolar disorder

  • in Parkinson’s

• Patients show altered circadian rest-activity rhythms, with fragmentation or phase shifts.

• Disrupted sleep may worsen psychiatric symptoms.

Explanation of Visuals:

• Actograms and sleep-wake data from patients compared to healthy controls.

• Visual differences show fragmented and less stable rhythms.

Glossary:

• Rest-activity rhythm: The pattern of movement and rest across the day.

• Circadian misalignment: Disruption between internal timing and the external environment.

Key Takeaway:
Disrupted circadian rhythms are a hallmark of many neurological and psychiatric disorders.

Slide 80: Chronobiological Therapies for Depression

Key Points:

• Chronotherapy uses controlled exposure to light, sleep timing, or melatonin to treat depression.

• Techniques include:

  • Sleep deprivation

  • Light therapy

  • Phase advance (adjusting sleep earlier)

• These therapies aim to resynchronize the circadian clock, improving mood and sleep.

Explanation of Visuals:

• Table summarizing different chronotherapeutic interventions, their combinations, and treatment outcomes.

Glossary:

• Chronotherapy: Treatment targeting the circadian system to alleviate mental health symptoms.

• Phase advance therapy: Gradual shifting of sleep timing earlier.

Key Takeaway:
Chronobiological interventions like light therapy and sleep timing adjustments are effective treatments for some depressive disorders.

Slide 81: Does the Circadian Clock Modulate Mood?

Key Points:

• Circadian rhythms influence mood and emotional behavior.

• Clock gene disruptions may affect mood regulation pathways in the brain.

• Animal models show altered behavior and neurochemical changes when clock genes are manipulated.

Explanation of Visuals:

• Clock and mood-related graphics.

• Bar graphs likely show differences in behavior or gene expression in animals under different circadian conditions.

Glossary:

• Clock-mood link: The idea that circadian timing mechanisms can affect emotional regulation and behavior.

Key Takeaway:
There is growing evidence that the circadian clock influences mood, and disturbances may contribute to mental health disorders.

Slide 82: Alcohol Consumption and Clock Genes

Key Points:

• Mice lacking Per1 or Per2 genes show increased alcohol consumption.

• Clock gene disruptions are linked to altered reward behavior and possibly addiction.

• This implies a role for the circadian system in regulating substance use.

Explanation of Visuals:

• Bar graphs show higher alcohol intake in Per1 and Per2 mutant mice.

• Circadian rhythm graph shows altered timing of alcohol preference.

Glossary:

• Per1/Per2: Core clock genes that regulate circadian rhythms.

• Reward behavior: Behavioral patterns related to seeking pleasurable stimuli like alcohol.

Key Takeaway:
Clock gene mutations can increase susceptibility to alcohol use, linking circadian regulation with addiction pathways.

Slide 83: Circadian Influence on Neurotransmitter Release

Overview:

This slide illustrates how the circadian clock gene Per2 influences glutamate neurotransmission in the brain and affects alcohol (EtOH) intake behavior in mice.

Diagram Explanation:

The slide compares two groups:

• Wild-type (normal) mice

• Per2 mutant mice (Per2^Brdm1), which have a disrupted circadian clock gene

In the synapse diagram:

• Presynaptic neurons release glutamate (a major excitatory neurotransmitter).

• Postsynaptic neurons receive this signal via glutamate receptors.

• Astrocytes (a type of glial cell) are shown reabsorbing glutamate from the synaptic cleft using glutamate transporters:

  • Eaat1 and Eaat2 (Excitatory amino acid transporters)

Key Observations:

• In wild-type mice, Eaat1 and Eaat2 transporters are expressed at normal levels, allowing efficient glutamate clearance from the synapse.

• In Per2 mutants, Eaat1 and Eaat2 expression is reduced, leading to:

  • Excess glutamate in the synaptic cleft

  • Prolonged stimulation of postsynaptic neurons

  • Possible alterations in reward and stress pathways

Behavioral Outcome (Mouse Icons Below):

• The wild-type mouse consumes a normal amount of alcohol (EtOH).

• The Per2 mutant mouse consumes significantly more alcohol.

• This suggests that the disrupted glutamate regulation (due to lack of proper Per2 expression) may drive increased alcohol-seeking behavior.

Scientific Interpretation:

• The circadian gene Per2 influences not just sleep-wake cycles but also glutamate homeostasis, affecting brain excitability and behavior.

• Dysregulation of clock genes like Per2 can impact addiction-related behaviors by altering neurotransmitter clearance dynamics.

Glossary:

• Glutamate: The main excitatory neurotransmitter in the brain.

• Astrocyte: A star-shaped glial cell that supports neurons and regulates neurotransmitter levels.

• Eaat1/Eaat2: Transport proteins that remove glutamate from the synaptic cleft.

• Per2: A core circadian clock gene involved in timing physiological processes.

Key Takeaway:

Disruption of circadian clock genes like Per2 impairs glutamate clearance at synapses, which can enhance excitatory signaling and promote behaviors such as increased alcohol consumption.

Let me know if you'd like this rewritten for a slide or simplified for a talk script.

Slide 84: Clock Gene Deficiency Affects Brain Systems

Key Points:

• Clock-deficient mice exhibit structural and functional changes in key brain areas.

• These include the hypothalamus, amygdala, and prefrontal cortex.

• Deficits in neurotransmitter systems may underlie behavioral and emotional changes.

Explanation of Visuals:

• Brain map highlights areas affected by clock gene mutations.

• Suggests a broad influence of circadian rhythms on brain development and function.

Glossary:

• Clock-deficient mice: Genetically modified mice lacking functioning circadian clocks.

• Amygdala/Prefrontal Cortex: Brain regions involved in emotion and executive function.

Key Takeaway:
Disruption of clock genes can impair major brain systems, potentially leading to mood and behavioral disorders.

Slide 85: Elevated Dopamine Levels in Per2 Mutant Mice

Key Points:

• Per2 mutant mice show elevated dopamine levels throughout the day.

• Disruption in the normal rhythm of dopamine may contribute to altered motivation and reward behaviors.

• This dysregulation supports the connection between circadian function and psychiatric conditions.

Explanation of Visuals:

• Graph shows significantly higher dopamine release in Per2 mutants compared to wild-type over 24 hours.

Glossary:

• Dopamine: A neurotransmitter involved in reward, motivation, and movement.

• Zetaplotter: (likely a mistyped or placeholder term for time axis representing hours in 24h format).

Key Takeaway:
Per2 mutations disrupt normal dopamine rhythms, linking the clock to mood and motivation circuits.

Slide 86: Circadian Clock Regulates Dopamine Breakdown (MAOA)

Key Points:

• The circadian clock controls expression of MAOA (monoamine oxidase A).

• MAOA degrades dopamine into inactive metabolites.

• Disruption of clock genes alters dopamine breakdown and can affect emotional regulation.

Explanation of Visuals:

• Diagram showing dopamine → MAOA → degradation product.

• The clock symbol emphasizes that this breakdown is under circadian control.

Glossary:

• MAOA: Enzyme that breaks down dopamine and other monoamines.

• Monoamines: A group of neurotransmitters including dopamine, serotonin, and norepinephrine.

Key Takeaway:
The circadian clock influences dopamine levels not just by synthesis, but also by regulating its breakdown via MAOA.

Slide 87: Chronopharmacology – Timing Matters

Key Points:

• Chronopharmacology is the study of how timing affects drug effectiveness.

• Drugs targeting the brain or body may work better at specific times due to circadian variation in metabolism and response.

• Understanding these rhythms can help optimize treatment schedules for better outcomes. The circadian rhythm has an impact on the drug one is taking.

Explanation of Visuals:

• Title and definition-style slide emphasizing the importance of drug timing.

Glossary:

• Chronopharmacology: The science of administering drugs in alignment with biological rhythms to maximize effect and minimize side effects.

Key Takeaway:
The effectiveness of drugs can vary by time of day—circadian-informed treatment timing can improve therapy.

Slide 88: Functions and Disorders Show a Circadian Rhythm

Key Points:

• DISORDERS HAVE ALSO CIRCADIAN RHYTHMS → Knowing about Circadian Rhythms are important to understand

• Many physiological functions and disease symptoms follow a circadian pattern.

• Examples include:

  • Heart attacks, asthma attacks → more common in the early morning.

  • Body temperature, hormone release, and blood pressure follow daily rhythms.

  • Rhythm means, at which timepoint does a drug reach the target fast through your blood → this might change on time.

• Chronotherapy aims to align treatment with these natural fluctuations.

Explanation of Visuals:

• A circular clock diagram shows the timing of different biological events and disorders across the 24-hour cycle.

Glossary:

• Chronotherapy: Timing treatments to match biological rhythms for better effect.

• Circadian Rhythm: A 24-hour biological cycle regulating behavior and physiology.

Key Takeaway:
Biological processes and many diseases show circadian variation—timing matters in therapy.

Slide 89: Five Disciplines of Chronopharmacology (Overview)

Key Points:

• Chronopharmacology includes five interrelated disciplines:

  • Chronokinetics

  • Chronodynamics

  • Chronotoxicology

  • Chronesthesy

  • Chronotherapy

• These areas investigate how timing affects drug actions, effects, toxicity, and therapeutic success.

Explanation of Visuals:

• Diagram showing five overlapping fields within chronopharmacology.

Glossary:

• Chronotoxicology: Study of time-of-day differences in drug side effects.

• Chronesthesy: Time-related differences in body sensitivity to a drug.

Key Takeaway:
Chronopharmacology is a broad field studying how biological rhythms influence drug action, toxicity, and effectiveness.

Slide 90: Definitions of the Five Disciplines of Chronopharmacology

Key Points:

• Chronokinetics/Chronodynamics: Timing influences drug absorption, metabolism, and action.

• Chronotoxicology: Side effects vary with time of administration.

• Chronesthesy: Target tissues vary in sensitivity based on time of day.

• Chronotherapy: Aligning treatment with biological rhythms improves outcomes.

• Chronopharmaceutics: Designing drug formulations for time-specific release.

Explanation of Visuals:

• Text-based definitions and distinctions between the five areas of chronopharmacology.

Glossary:

• Pharmacokinetics: How the body processes a drug (ADME: Absorption, Distribution, Metabolism, Excretion).

• Pharmacodynamics: How the drug affects the body.

Key Takeaway:
Chronopharmacology is multidimensional—each aspect helps optimize drug effects and reduce side effects through time-based strategies.

Slide 91: Determinants of Pharmacokinetics (ADME)

Key Points:

• ADME: Four key steps in drug processing:

  • Absorption: Entry into the bloodstream.

  • Distribution: Delivery to tissues/organs.

  • Metabolism: Chemical modification (often in liver).

  • Excretion: Removal from the body (via kidneys, bile, etc).

• Each step can be influenced by circadian rhythms.

Explanation of Visuals:

• Flow diagram showing how a drug is processed from absorption to excretion.

Glossary:

• Pharmacokinetics: The movement of drugs through the body.

• Excretion: The process of eliminating waste substances, including drugs.

Key Takeaway:
The body’s handling of drugs (ADME) follows a time-dependent pattern and affects drug effectiveness and safety.

Slide 92: Chrono PK – Absorption

Key Points:

• Drug absorption is affected by pH, GI motility, and blood flow, which vary with time of day.

• Examples:

  • Lipophilic drugs (fat-soluble) are better absorbed in the morning.

  • Some antiepileptics (e.g. valproic acid) are better absorbed in the evening.

  • Skin permeability also follows a rhythm → influences topical drug absorption.

Explanation of Visuals:

• Diagrams of the gastrointestinal system and drug examples demonstrate time-of-day effects on absorption.

Glossary:

• Lipophilic drugs: Fat-soluble drugs that depend on GI conditions for absorption.

• Gastric emptying: The speed at which food and drugs leave the stomach.

Key Takeaway:
Absorption of many drugs varies with circadian-controlled changes in GI physiology—morning and evening can yield different outcomes.

Slide 93: Chrono PK – Distribution

Key Points:

• Blood flow distribution varies with time of day, especially due to circadian regulation of the autonomic nervous system.

• Drug binding to plasma proteins (like albumin) is also time-dependent.

• As a result, the free (active) drug fraction can vary over the day.

Examples:

• Propranolol, diazepam, and valproic acid show time-dependent differences in how they are distributed in the body.

Explanation of Visuals:

• Diagrams of the cardiovascular and systemic circulation emphasize rhythmic variation in drug delivery and plasma protein interaction.

Glossary:

• Free drug fraction: The portion of a drug not bound to proteins—active in the body.

• Sympathetic nervous system: Regulates blood flow and other circadian-driven functions.

Key Takeaway:
Drug distribution is influenced by circadian changes in blood flow and protein binding, affecting efficacy and toxicity.

Slide 94: Chrono PK – Metabolism

Key Points:

• Liver metabolism depends on both enzymatic activity and hepatic blood flow:

  • High-extraction drugs: More influenced by blood flow.

  • Low-extraction drugs: More dependent on enzyme activity.

• Blood flow to the liver peaks in the morning, while enzyme activity is often higher at night.

• This leads to time-of-day differences in how drugs are metabolized.

Glossary:

• Hepatic metabolism: The liver’s processing of drugs through enzyme activity.

• Extraction ratio: Proportion of drug cleared from the blood by the liver.

Key Takeaway:
Drug metabolism follows circadian patterns influenced by liver function—timing drug intake can change drug effectiveness.

Slide 95: Chrono PK – Elimination

Key Points:

• Kidney functions (filtration, urine flow, blood flow) are rhythmic.

• Most renal functions peak during the day → drugs are excreted faster.

• This affects drug clearance and can require higher nighttime doses to maintain levels.

Glossary:

• Glomerular filtration rate (GFR): A measure of kidney function.

• Urinary pH: Affects solubility and excretion of drugs.

Key Takeaway:
Drug elimination by the kidneys is more active during the day, impacting how long drugs stay in the body depending on administration time.

Slide 96: Chronopharmacology Can Improve Therapies

Key Points:

• Aligning drug timing with circadian rhythms can:

  • Increase efficacy (e.g., statins work better in the evening).

  • Reduce side effects.

  • Improve survival (e.g., cancer chemotherapy at the right time).

• Example: Evening chemotherapy for leukemia is associated with lower mortality than morning treatment.

Explanation of Visuals:

• Graph shows better outcomes when drugs are chronotherapy-aligned.

Glossary:

• Chronotherapy: Timing treatment to a patient’s internal clock for better outcomes.

Key Takeaway:
Chronopharmacology can make existing treatments more effective and safer just by adjusting the time of administration.

Slide 97: Chrono-tailored Drug Delivery Systems

Key Points:

• Circadian timing should be integrated into drug delivery systems.

• Options include:

  • Controlled-release pills timed for specific biological rhythms.

  • Devices or formulations that adapt to body clocks.

• Such systems can maximize therapeutic impact and reduce toxicity.

Explanation of Visuals:

• Diagram shows organs affected by circadian rhythms and potential sites for timed drug delivery.

Glossary:

• Sustained/controlled release: Drug delivery methods that release medication over time.

Key Takeaway:
Modern medicine is developing smart delivery systems that match drugs to the body’s natural clock for optimized results.

Slide 98: Summary – Circadian Rhythms

Key Points:

• We are synchronized by our circadian rhythms

• we need light in order to be synchronized

• Only about 10% of our genes that are transcribed into the mechanisms that manage these rhythms for us.

• The central clock in the brain and peripheral clocks in organs coordinate:

  • Hormone release

  • Sleep/wake cycles

  • Body temperature

  • Metabolism

• These clocks regulate core genes and physiological processes via transcriptional feedback loops.

Explanation of Visuals:

• Diagram shows clock components in the brain and organs, including feedback loops (e.g., PER, CRY genes).

Glossary:

• Suprachiasmatic nucleus (SCN): The master clock in the brain.

• Transcriptional loop: A gene regulation cycle involving activation and repression over 24 hours.

Key Takeaway:
Circadian clocks are deeply embedded in body systems, regulating gene expression and physiology across multiple organs.

Slide 99: Summary – Chronopharmacology

Key Points:

• Chronopharmacology links the circadian rhythm to drug processing steps:

  • Absorption

  • Distribution

  • Metabolism

  • Excretion

• Timing medication can help target the right tissues at the right moment for maximum efficacy and minimal harm.

Explanation of Visuals:

• Summary diagram connects circadian timing to key organs responsible for drug pharmacokinetics (gut, liver, kidneys, etc.).

Glossary:

• Chronopharmacology: Study of how time of day affects drug action and metabolism.

Key Takeaway:
Understanding the body’s rhythms allows more precise, personalized, and effective drug administration—“when” you take a drug matters.