(PTS Sem2) Sleep and Biological Rhythms

What is Sleep?

  • A state generated by the brain, characterized by reduced brain activity and responsiveness to external stimuli.

Biological Rhythms:

  • Tools to detect sleep brain activity:

    • EEG (electroencephalography): measures brain activity, helping scientists study sleep phases.

    • polysomnography: more advanced tool that combines EEG with eye movement tracking to provide more detailed information about your sleep

  • Alpha waves: specific brain wave patterns seen when you’re relaxed but not yet asleep, often observed when you are calm or meditating

  • Sleep phases

    1. Sleep Phases: REM(Rapid Eye Movement Sleep): The Dreaming Stage

      *important for memory, learning, and emotional processing.

      • Rapid Eye Movements: The eyes move quickly under the eyelids.

      • "Paradoxical Sleep": This phase is strange because the brain is very active (like when awake), but the body is deeply relaxed.

      • Brain Waves: EEG shows fast, low-voltage, and irregular waves, similar to when a person is awake.

      • Muscle Relaxation: The body is more relaxed (almost paralyzed) than in any other sleep stage. This prevents people from acting out their dreams.

    2. Sleep Phases: Non-REM(all sleep stages besides REM

      *important for rest, recovery, and memory processing

      • Non-REM (NREM) Sleep: All sleep stages except REM are called NREM sleep.

      • Falling Asleep: First, a person moves through Phase 1 → Phase 2 → Slow-Wave Sleep (Deep Sleep) in order.

      • Reversing Back: After about one hour, the process goes backward from deep sleep (SWS) → Phase 2 → then enters REM sleep (the dreaming stage).

      • Repeating the Cycle: This 90-minute cycle happens multiple times throughout the night, with REM sleep lasting longer in later cycles

  • Stages of non-REM sleep based on EEG activity:

    1. Phase 1 (Light Sleep/Beginning of Sleep)

      • Brain waves are irregular and have low voltage.

      • Brain activity starts slowing down as the person begins to fall asleep.

    2. Phase 2 (Deeper Sleep but Not Deep Yet)

      • The brain shows specific wave patterns:

        • K-complexes: Sudden large waves that help keep the person asleep.

        • Sleep spindles: Quick bursts of brain activity (12-14 Hz) that help with memory and learning.

        *These waves are linked to intelligence and memory retention

    3. Phases 3-4 (Deep Sleep/Slow-Wave Sleep - SWS)

      • Brain waves become much slower and larger.

      • The heart rate, breathing rate, and brain activity slow down.

      • Neurons fire in a highly synchronized manner, allowing deep rest and recovery.

  • Sleep Phases: Facts

    • Deep Sleep (SWS) Happens More at the Start: In the first few hours, you spend more time in Slow-Wave Sleep (SWS), which is the deepest and most restful sleep.

    • PGO waves: high-amplitude brain waves that occur during REM sleep (dreaming stage);

      PGO waves → Travel from the brainstem to the visual processing area → Help create dream visuals

      *These waves are linked to visual imagery in dreams and may explain why dreams often feel so vivid

    • Less Deep Sleep Later: As the night goes on, SWS gets shorter and happens less often.

    • More REM Sleep in the Morning: REM sleep (dreaming stage) becomes more frequent and lasts longer as morning approaches.

    • Dreams Can Happen in Any Stage: Most dreams occur in REM sleep, but people also report dreams in other sleep phases too.

    *This pattern helps the body recover early in the night and focus on memory and learning later

Neural Mechanisms in Sleep:

  1. Brain Mechanisms Controlling Wakefulness & Alertness:

    • Reticular Formation (Midbrain to Forebrain) → Controls arousal & wakefulness

      • Extends from the medulla to the forebrain

      • Helps regulate alertness by influencing brain activity

    • Pontomesencephalon (Part of Midbrain) → Maintains cortical arousal

      • Sends axons to the thalamus & basal forebrain

      • Releases acetylcholine & glutamate → Excites large areas of the cortex

      • Stimulation wakes up a sleeping person & increases alertness in someone already awake

    • Locus Coeruleus (Pons) → Enhances alertness & attention

      • Small structure that releases norepinephrine

      • Increases alertness by stimulating areas of the cortex

      • Inactive during sleep

    • Orexin (Lateral Hypothalamus) → Maintains wakefulness & alertness

      • Stimulates acetylcholine-releasing neurons in the basal forebrain

      • Helps sustain wakefulness & prevent sudden sleep episodes

      • Orexin deficiency leads to unstable sleep-wake cycles (e.g., falling asleep during activities)

      • Orexin-blocking drugs can help treat insomnia

  2. How the Brain Prepares for Sleep

    • Thalamus (Sensory Relay Center) Slows Down

      • Reduces sensory input to the cerebral cortex → Harder to wake up

      • Becomes hyperpolarized, making it less responsive to weak stimuli (but strong stimuli can still wake a person up)

    • GABA Release Increases → Reduces Brain Activity & Induces Sleep

      • Inhibitory neurotransmitter that weakens neural connections

      • Lowers body temperature & metabolic rate

      • Reduces neuronal stimulation, making it easier to stay asleep

  3. Local Sleep Phenomena (When Some Brain Areas Are Awake & Others Are Asleep)

    • Sleepwalking (Somnambulism)

      • The motor cortex remains active, allowing movement

      • Areas responsible for decision-making (prefrontal cortex) remain inactive → Actions are automatic & can be risky

    • Lucid Dreaming

      • Some brain regions stay awake, leading to awareness of dreaming

      • Allows conscious monitoring & control of dreams while still in a sleep state

    *Sleep is a delicate balance between brain regions responsible for arousal and inhibition, regulated by neurotransmitters like GABA, acetylcholine, norepinephrine, and orexin.

Functions of Sleep: Energy Conservation, Brain & Body Restoration, and Memory Consolidation

Sleep Disorders:

  • Insomnia: Insufficient sleep leading to tiredness and cognitive impairment.

  • Sleep Apnea: Breathing interruptions during sleep that can severely affect overall health.

  • Narcolepsy: Excessive daytime sleepiness and sudden sleep attacks linked to orexin deficiencies.

Cycles regulate bodily functions, such as sleep-wake cycles, and include:

  1. Circadian rhythms: internal(endogenous) body clock that follows a 24-hour cycle for sleep and bodily functions(hunger, body temperature, etc)

  2. Infradian rhythms: the occurrence is Longer than a day, e.g., menstrual cycle.

  3. Ultradian rhythms: the occurrence is shorter than a day but repeats multiple times.

1.1 Circadian Rhythms:

  • What is Circadian Rhythms

    • Operate in cycles close to 24 hours and regulate many processes:

      - Influences wakefulness, hormone secretion, body temperature, urination, sensitivity to drugs, and even mood.

  • Functions of Circadian Rhythms

    • Circadian rhythms help the body stay in sync with the outside world, like day and night.

    • ways body tries to adapt:

      1. Free-running rhythm: The body's natural cycle when there are no external clues (like light) to reset it.

      2. Zeitgeber: Anything that resets the body clock. Light is the most important one, but exercise, noise, eating, and temperature also help.

  • Circadian Rhythm Disorders:

    1. Jet Lag: Disruption from crossing time zones

      • steams from a misalignment between the internal circadian clock and external time

      • symptoms like insomnia and fatigue.

      • lead to increased levels of the stress hormone cortisol, and in severe cases, it may damage hippocampal cells.

    2. Shift Work Disorders: Poor cognitive performance linked to misaligned circadian rhythms. caused by working at odd hours such as night shift

  • Individual Variations in Sleep Patterns:

    • Influenced by age, genetics, and environmental factors(e.g., artificial lighting)

    • Children sleep earlier and wake up earlier, while teenagers tend to sleep later and wake up later due to the influence of sex hormones

    • Sleep shift difficulty:

      • People can adjust to days that are a little shorter/a little longer but they struggle with bigger changes because it’s too far from their natural rhythm

  • Circadian Mechanisms:

    1. Suprachiasmatic Nucleus (SCN)

    2. Melatonin levels

    3. Genes producing specific proteins

    4. Neural processes in the brain

  1. Suprachiasmatic Nucleus(SCN):

A cluster of thousands of cells in the hypothalamus regulates wakefulness and sleep by controlling different brain areas;

*SCN cells work together to make the body's circadian rhythm stronger and more precise.

  • SCN regulates rhythms genetically, responding primarily to light input.

  • important genes that help control the body's internal clock:

    1. Period (Per) gene → Makes PER protein

    2. Timeless (Tim) gene → Makes TIM protein

      • help SCN neurons control sleep and wake cycles

      • If the Per gene mutates, it can disrupt the circadian rhythm, making it shorter or longer than 24 hours, causing sleep problems.

  • SCN cycle works in response to light:

    Light exposure → detected by intrinsically photosensitive retinal ganglion cells (ipRGCs) (not rods or cones) → Melanopsin in ipRGCs absorbs light → Signal sent through the retinohypothalamic tract → Reaches the SCN in the hypothalamus → SCN adjusts the circadian rhythm to match the light-dark cycle.

    • retinohypothalamic tract: small branch of the optic nerve(special pathway)

    • intrinsically photosensitive retinal ganglion cells (ipRGCs): specialized retinal ganglion cells

    • Melanopsin: Melanopsin is a light-sensitive pigment found in specialized retinal ganglion cells. These cells send signals to the SCN to help regulate the body's circadian rhythm.

      *melanopsin-containing cells detect overall light levels to help reset the biological clock.

  • Damage to the SCN results:

    • less consistent body rhythms

    • a loss of synchronization with the light-dark cycles of the environment

  1. Melatonin levels

    • Melatonin: hormone that helps regulate sleep. It is produced by the pineal gland in response to darkness, signaling to the body that it’s time to sleep

    • SCN’s role in melatonin regulation:

      SCN (body’s clock) → Sends signals to the pineal gland → Pineal gland releases melatonin → Melatonin makes you feel sleepy → More melatonin is released in the dark, helping you sleep

    • The body starts releasing melatonin 2–3 hours before bedtime to help you feel sleepy.

    • Melatonin helps reset the body's clock by affecting the SCN.

    • Taking melatonin during the day can shift the sleep schedule earlier (phase-advance), which is why it’s sometimes used as a sleep aid

Comparative States of Consciousness:

  • Coma: Prolonged unconsciousness with low brain activity.

  • Vegetative State: Alternates between sleep and arousal but lacks awareness.

  • Minimally Conscious State: a state of unconsciousness where there are brief moments of showing signs of consciousness such as brief noises, movements, and so on

  • Brain Death: Complete absence of brain activity and responsiveness.

Sleep Disorders

  • Insomnia

    • Caused by genetic, environmental, or psychological factors.

    • Sleep deprivation affects memory, attention, emotions, and increases depression risk.

    • Misaligned circadian rhythm and caffeine can worsen it.

  • Sleep Apnea

    • Breathing disorder → stops breathing for minutes, gasping for air.

    • Reduces sleep quality, increases risks of stroke and heart disease.

    • Oxygen deprivation damages brain cells.

    • Risk factors: genetics, hormones, age, obesity.

    • Many are unaware of their disrupted sleep; often noticed by others

  • Narcolepsy

    • Causes excessive daytime sleepiness.

    • Symptoms: sudden sleep episodes, muscle weakness, sleep paralysis, hallucinations.

    • Linked to orexin deficiency (neurotransmitter for wakefulness).

Dreams

  • Activation-Synthesis Hypothesis

    • Dreams = brain’s attempt to interpret random neural activity.

    • Spontaneous signals from the pons activate parts of the cortex → incomplete, distorted narratives.

    • Example: Inactivity of vestibular system → floating sensation → dreams of flying or falling

  • Neurocognitive Hypothesis

    • Dreams = form of thinking in an unusual brain state.

    • Spontaneous brain activity triggers memories & visualizations.

    • Limited sensory input → brain freely generates images.

    • Emotional parts of the brain (amygdala & hypothalamus) highly active, explaining emotional dream content.