Biological Rhythms & Sleep Study Notes

Biological Rhythms – Core Ideas

  • Biological rhythm = any regular, cyclic fluctuation in a living process.

    • Keeps physiology & behavior synchronized with environmental cycles.

    • Measured in virtually every taxon (plants, fungi, animals, bacteria).

  • Three main temporal scales

    • Circadian (“about a day”) ≈ 24 h24 \text{ h}.
      • Human rest/activity, body temperature, hormone secretion (e.g., cortisol peaks A.M.), drug sensitivity, rodent running or drinking.

    • Ultradian (< 24 h; multiple cycles per day).
      • Feeding bouts, pulsatile hormone release, mammalian sleep cycles 90–110 min90–110\text{ min}.

    • Infradian (Less frequent then once a day)
      • Menstrual & breeding cycles, seasonal migration, hibernation.

Endogenous Circadian Clock

  • Humans = diurnal: active during the day

  • many mammals = nocturnal: active during the night → activity restricted by an internal clock, not an alarm.

  • Wheel-running assay (Figure 10.2)

    • Rodent in light-dark (LD) 12:12 shows entrained running in the dark.

    • When switched to constant dim light (LL) → rhythm free-runs with a stable but slightly shorter/longer period.

    • Phase shift in LD schedule instantly shifts onset of activity → since the lights are coming on less frequently, they change when they go to sleep 5

  • Human free-run: rhythm without cues

  • Entrainment: the process by which an animal slowly shifts its circadian rhythm to match with the sunrise each day

Suprachiasmatic Nucleus (SCN)

  • where the biological clock is located

  • above optic chiasm in hypothalamus.

    • Master pacemaker: drives sleep, core body temperature, hormonal & peripheral (liver, heart, lung) clocks.

    • Synchronizes every cell’s molecular clock.

  • Retinohypothalamic pathway: carries info from the eyes to the thalamus

    • Specialized melanopsin-containing retinal ganglion cells (RGCs) → monosynaptic projection to SCN.

    • Circadian rhythms are entrained by light

    • Light = Zeitgeber ("time-giver").

  • SCN → pineal gland → melatonin

    • SCN synapses at the pineal gland

    • Light inhibits pineal.

    • Pineal creates melatonin

    • Less Light = more melatonin

SCN Lesion & Transplant

  • Lesion the hamsters SCN→ abolishes circadian drinking, locomotion, hormone rhythms (arrhythmic).

  • Transplant from mutant hamster (period ≈ 20 h20\text{ h}) into lesioned wild-type → recipient adopts donor’s shorter rhythm ⇒ rhythm generated in SCN.

Sleep Across the Lifespan

  • Puberty delay: circadian phase shifts later; high-school start ≥ 08:30 → ↑ grades, ↓ depression, 70 % fewer car crashes.

  • Infants

    • Total sleep ≈ 16 h; cycles ≈ 50 min; REM ≈ 50 % (developmental stimulation).

  • Aging

    • Total sleep ↓, awakenings ↑.

    • Stage 3 (SWS) ↓ by 50 % at 60 y, effectively gone by 90 y.

  • Women vs Men

    • Meta-analyses: women need ≈11–20 min more sleep, linked to hormonal cycling, mental health, caregiving load.

  • National Sleep Foundation ranges (college-age 18–25 y): 7–9 h7–9\text{ h} recommended, 6–11 h6–11\text{ h} may be appropriate.

Sleep Architecture & EEG Markers

  • EEG: record electrical activity in the brain, can be used to classify sleep

    2 kinds of sleep

  • Rapid eye movement (REM): small aplmplitufe, fast eeg waves,

  • Non-REM: three stages with EEG waves

  • Wakefulness

    • Beta (13–30 Hz): low-amp/high-freq, “desynchronized”.

    • Eyes closed → Alpha (8–12 Hz), relaxed.

  • Stage 1 (N1)

    • HR ↓, muscle relax; less alpha, irregular theta;

    • vertex spikes.

    • lower amp

    • If awoken, deny being asleep.

  • Stage 2 (N2)

    • Sleep spindles (12–14 Hz bursts) + K-complexes (high-amp biphasic).

    • If awoken, deny being asleep

  • Stage 3 (N3, SWS)

    • Delta waves (0.5–4 Hz) high-amp, synchronous.

    • “Deep sleep”.

  • REM (paradoxical)

    • Low-amp/high-freq like wake; rapid eye movements; atonia (spinal motoneurons suppressed); irregular HR/resp.

    • Vivid, narrative dreaming.

  • Typical 7–8 h night

    • 4–5 cycles, 90–110 min90–110\text{ min} each.

    • Early cycles = more SWS, late cycles = longer REM (~20 % of night).

    • Mostly in stage 2

  • Dream facts:

    • Most story like in REM sleep

    • Activation-synthesis hypothesis: Dreams are the cortex’s attempt at making sense of random brain activity

    • External cues can enter dreams.

    • Sleepwalking rare in REM (due to atonia).

  • Nightmares (REM): long frightening dreams that wake someone from REM sleep

  • vs Night terrors (SWS) → sudden arousals during stage 3, slow waves sleep, fear and autonomic activity

Functions of Sleep

  • Energy conservation: ↓ muscle tone, HR, BP, core T, respiration.

  • Niche adaptation: sleep during vulnerable periods, helps to avoid predators

  • Restoration:

    • Replenish proteins, clear waste (glymphatic flow).

    • Growth hormone released during SWS; supports immune function.

  • Memory consolidation:

    • REM → perceptual & emotional memories.

    • SWS → declarative + complex motor skills.

    • But REM not strictly required for learning.

Neural Control of Sleep – Four Interacting Systems

  1. Forebrain: generates SWS

    • GABAergic; induces SWS.

    • Stimulate → SWS;

    • lesion → insomnia.

  2. Brainstem: activates forebrain in wakefulness

    • Diffuse ACh, NE, 5-HT arousal; promotes wakefulness.

    • Lesion → persistent sleep;

    • stimulation → waking EEG.

    • Reticular formation promotes wakefulness

  3. Pontine: triggers REM smell

    • REM-on neurons; inhibit spinal motoneurons → atonia.

    • Lesion below locus coeruleus → animals “act out” REM.

    • Stimulate → trigger/prolong REM.

    • Area of the pons called the subcoeruleus is responsible for REM sleep

    • Inhibits motor neurons during sleep

  4. Hypothalamic switch (lateral & tuberomammillary nuclei; orexin neurons)

    • Coordinates transitions among wake, NREM, REM.

    • SCN supplies circadian timing; hypothalamus executes state shifts.

Sleep Disorders

  • Narcolepsy

    • Frequent sleep attacks; direct REM onset

    • episodes last (≤ 5 min),

    • no preceding SWS.

    • Cataplexy: sudden loss of muscle tone without loss of consciousness leading to collapse

    • Linked to orexin loss.

  • Sleep Disorders are more common in children

    • Night terror:intense fear and autonomic activation

    • sleep enuresis: bed wetting (stage 3)

    • somnambulism: sleepwalking, l(activity mimics day’s tasks).

    • Most kids outgrow.

  • Insomnia (10–40 % adults)( most common in adult women)

    • Sleep-onset: trouble falling asleep (shift work, jet lag).

    • Sleep-maintenance: trouble staying asleep (drugs, psych disorders).

    • More common in older adults & women.

  • Sleep apnea: inability to breathe while sleeping

    • Repeated breathing cessation; obstructive vs central.

    • Risks: obesity, large adenoids, brainstem dysfunction.

    • CPAP therapy.

  • Sleep state misperception: perceive sleeplessness despite EEG sleep.

  • REM behavior disorder (RBD)

  • When you act out your dreams, no muscle atonia

    • Loss of atonia → complex movement during REM; ↑ in men > 50.

    • Pontine damage; treated with bedtime benzodiazepines.

  • Sudden infant death syndrome (SIDS)

    • Immature respiratory control; “Back to Sleep” reduces risk.

    • Stops breathing during sleep

Sleep Deprivation & Recovery

  • Total/partial deprivation → ↑ irritability, ↓ concentration, disorientation, slow reaction time, microsleeps (seconds-long). (enter stage 1)

  • Randy Gardner (1964): 11 d awake; recovery nights show SWS rebound > REM rebound; never fully “pay back” debt.

  • Sleep recovery: sleeping more than usual after a time of deprivation, never make up ALL that was lost

Pharmacology & Behavioral Sleep Aids

  • Hypnotic drugs (benzodiazepines, Z-drugs)

    • Most sleeping pills binds to GABA receptors .

    • Alter architecture (↓ SWS & REM), tolerance develops, daytime drowsiness, memory gaps.

  • Exogenous melatonin

    • Mimics darkness signal; useful for jet lag & blindness.

    • Promotes drowsiness but not an absolute hypnotic; advisable short-term.

  • Sleep hygiene

    • Consistent wake time, bed only when sleepy.

    • No naps, limit caffeine post-noon.

    • Cool, dark, quiet room; screen curfew (blue light ↓ melatonin).

Emotion, Neurochemistry & Sleep

  • Sleep loss → hyper-reactive amygdala & weakened PFC-amygdala connectivity → irritability (Goldstein & Walker 2015).

  • Neurotransmitters

    • Dopamine (DA) & norepinephrine (NE) ↑ arousal; GABA ↑ sleep.

    • Anxiety/excitement (↑DA/NE) fights sleep; boredom (↓DA/NE) invites sleep.

Oversleeping & Grogginess

  • Sleep inertia: waking from deep SWS → impaired alertness.

  • Oversleeping disrupts circadian timing; often accompanies poor-quality, fragmented sleep.

  • Adenosine accumulates during wake, cleared in sleep; excessive sleep may leave residual adenosine → groggy.

Dream & Consciousness Research Highlights

  • Lucid dreaming via tDCS: frontal gamma (40 Hz) stimulation → self-awareness in REM (Voss 2014).

  • Recurrent dreams track unresolved waking issues; resolution stops recurrence (Zadra 1996).

  • Real-time REM communication: participants under REM answered math Qs using eye/facial codes (Konkoly 2021).

Suggested Enrichment

  • TED Talks:
    • Jeff Iliff – glymphatic clearance.
    • Russell Foster – evolutionary purpose.
    • Matt Walker – “Sleep is your superpower” & “6 Tips for better sleep”.