Behavioral Neuroscience - Circadian Rhythms

Sleep Biorhythms 1

Definition: Circadian rhythms are functions of living organisms that display a rhythm of approximately 24 hours. They encompass physical, mental, biochemical, and behavioral changes in organisms, adapting to the day-night cycle.

Diurnal: Active during daylight hours. Examples include humans, dogs, and elephants.
Nocturnal: Active during the night. Examples include bats and skunks.
Crepuscular: Active during dusk and dawn. Examples include house cats, rabbits, and deer.

Endogenous Clock: Internal timing mechanism responsible for generating circadian rhythms.
Zeitgeber: A German term meaning “time giver.” Light is a primary zeitgeber that helps synchronize the endogenous clock. Other cues, like food, can also serve as zeitgebers.
- Reference: Evidence can be found in studies such as those available at ResearchGate.

Hamsters are nocturnal animals that display behaviors based on light and dark cycles.
Behavioral Observations:
- Hamsters show increased activity on running wheels during dark periods and sleep during light periods.
- Changing light conditions causes phase shifts in activity patterns (e.g., lights turning on later results in later activity).
- Under constant dim light, hamsters exhibit a free-running cycle that is approximately 25 hours.

When the primary zeitgeber of changing light conditions is removed, animals become free-running, following their internal clocks.
Phase Shift Example: This is demonstrated through activity plots where hamsters modify their activity patterns in response to shifts in lighting.

Location: SCN is located in the hypothalamus, directly above the optic chiasm.
Function: Responsible for regulating circadian rhythms; SCN lesions disrupt these rhythms.
Frankenhamster Study: Research using genetically modified hamsters (tau mutation) demonstrated that rhythm lengths can be altered by SCN transplants. Double tau hamsters exhibit rhythms of 20 hours; normal hamsters typically have a rhythm of 25 hours.

Retinohypothalamic Pathway: Transmits light information from the retina to the SCN, distinct from the visual processing pathways.
ipRGCs (Intrinsically Photosensitive Retinal Ganglion Cells): Specialized ganglion cells that contain melanopsin, making them sensitive to light, particularly blue light, which plays a role in entraining the SCN.

Even in the absence of rods and cones, blind individuals and animals can maintain normal circadian rhythms due to the functioning of melanopsin-containing ipRGCs that continue to send light information to the SCN.

Molecular Clock: The SCN's timing mechanism is regulated through the synthesis and degradation of specific proteins.
Key Proteins:
- Clock and Cycle (with BMAL1 being the mammalian variant) form a dimer to activate the transcription of period (PER) and cryptochrome (CRY) proteins.
- The production of PER and CRY results in negative feedback inhibition on Clock/Cycle, creating a cycle of approximately 24 hours.

Chronotype: An individual's natural preference regarding sleep and wake times.
Genetic differences among the per (period) genes are observed between “larks” (morning types) and “night owls” (evening types).
Assessment Tool: The Morningness–Eveningness Questionnaire (MEQ) assesses chronotypes.

Geographical Influence: People living on the western edge of time zones tend to go to bed later than those on the eastern edge, influenced by the patterns of sunlight.
Daily bedtime often deviates from natural light patterns due to societal and personal factors.

Infradian Rhythms: Biological rhythms that occur less than once per day, e.g., reproductive cycles.
Ultradian Rhythms: Biological rhythms that occur more than once per day, e.g., feeding patterns, bouts of activity, and hormone releases.
Circannual Rhythm: Example includes rhythms that have an approximate yearly (365 days) period, observed in seasonal animals, which display free-running annual rhythms, influenced by different mechanisms than the SCN involving melatonin.

Light information suppresses melatonin secretion by the pineal gland, allowing light cues to regulate various biological functions.
Seasonal Changes: During prolonged dark periods (e.g., winter), increased melatonin production influences seasonal behavior, although the exact mechanisms remain unclear.

Significance: While all light can suppress melatonin secretion, blue light has a more substantial effect.
Harvard Study Findings: Exposure to blue light for 6.5 hours resulted in nearly double the melatonin suppression compared to green light and shifting in circadian rhythms—3 hours for blue light, versus 1.5 hours for green light.
- Source Reference: Detailed study findings can be accessed through Harvard Health.