Normal Sleep Physiology

Circadian Rhythms & Sleep Basics - Tick-Tock Sleep

• Sleep: A reversible state of reduced responsiveness, motor activity, and metabolism, crucial for restoration and memory consolidation.
• Circadian Rhythm: An endogenous biological clock with a cycle of approximately 24 hours, governing the sleep-wake pattern.
  • Master clock: Suprachiasmatic Nucleus (SCN) in the hypothalamus.
  • Primary synchronizer (zeitgeber): Light, which entrains the SCN.
• Key Regulators:
  • Melatonin: Hormone from the pineal gland; its secretion ↑ in darkness, promoting sleep.
  • Adenosine: Accumulates during wakefulness, promoting sleep drive (Process S).
• Two-Process Model of Sleep Regulation:
  • Process S (Homeostatic): Sleep pressure builds with prolonged wakefulness.
  • Process C (Circadian): Oscillating alerting signal from the SCN.

⭐ The SCN controls the timing of sleep but not sleep generation itself; SCN lesions disrupt sleep timing but not total sleep amount over 24 hours.

Sleep Architecture & Stages - Night's Blueprint

• Sleep Cycle: 4-6 cycles/night, each ~90-120 minutes. Alternates NREM & REM.
• Polysomnography (PSG): Key tool; records EEG (brain), EOG (eyes), EMG (muscles).
• NREM Sleep (Non-Rapid Eye Movement; ~75-80%): "Quiescent brain, active body".
  • N1 (5-10%): Transition to sleep; light sleep.
    • EEG: Theta waves ($4-7$ Hz), vertex sharp waves. EOG: Slow rolling eye movements.
  • N2 (45-55%): True sleep onset; majority of sleep.
    • EEG: Sleep spindles ($12-14$ Hz), K-complexes.
  • N3 (15-25%): Slow Wave Sleep (SWS); deep, restorative sleep.
    • EEG: Delta waves ($0.5-2$ Hz, >75 μV amplitude).
    • Physiology: ↑Growth Hormone release, ↓cortisol. 📌 "Delta for Deepest".
• REM Sleep (Rapid Eye Movement; ~20-25%): "Active brain ('paradoxical sleep'), paralyzed body".
  • EEG: Low voltage, mixed frequency; characteristic "sawtooth" waves.
  • EOG: Rapid, conjugate eye movements.
  • EMG: Muscle atonia (except diaphragm, extraocular muscles).
  • Physiology: Vivid dreaming, memory consolidation. ↑Brain O₂ consumption.

  ⭐ Most REM sleep occurs in the latter half of the night, with REM periods becoming longer and more frequent as sleep progresses.

Neurobiology of Sleep - Brain's Off Switch

• Key Brain Regions & "Switches":

  • SCN (Suprachiasmatic Nucleus): Master circadian clock (hypothalamus); light-sensitive.
  • Hypothalamus:
    • VLPO (Ventrolateral Preoptic Nucleus): "Sleep switch"; GABAergic.
    • LHA (Lateral Hypothalamic Area): "Wake switch"; Orexin/Hypocretin.
  • Brainstem:
    • ARAS (Ascending Reticular Activating System): Maintains wakefulness (ACh, NE, 5-HT, DA, Histamine).
    • Pons (LDT/PPT): REM sleep generation (ACh).
  • Thalamus: Sensory gate during sleep.

• Key Neurotransmitters:

  • Sleep-Promoting:
    • Adenosine: ↑ with wakefulness, ↑ sleep pressure.
    • GABA: Major inhibitory NT (e.g., VLPO).
    • Melatonin (Pineal): Circadian rhythm, sleep onset.
  • Wake-Promoting:
    • Orexin/Hypocretin: Stabilizes wakefulness.
    • ACh (Acetylcholine): Arousal; ↑ in wake & REM.
    • NE (Norepinephrine), 5-HT (Serotonin), DA (Dopamine), Histamine: Promote arousal.

⭐ Orexin (Hypocretin) deficiency, produced in the lateral hypothalamus, is the primary cause of Type 1 Narcolepsy, highlighting its critical role in sustaining wakefulness.

Sleep Functions & Age Variations - Purpose Through Time

• Functions: Restoration (body/brain), memory consolidation, energy conservation, immune regulation.
• Age Variations:
  • Infants: ~16-18 hrs/day; ~50% REM; sleep cycles ~50 min.
  • Children/Adolescents: ↓ total sleep, ↓ REM %; adolescent phase delay.
  • Adults: Stable; SWS (N3) gradually ↓.
  • Elderly: ↓ total sleep time, ↓ SWS (N3), ↑ WASO (Wake After Sleep Onset), ↑ fragmentation; phase advance common.

  ⭐ Neonates have the highest REM sleep proportion (~50% of total sleep), vital for brain development.

High‑Yield Points - ⚡ Biggest Takeaways

• Sleep comprises NREM (N1, N2, N3/SWS) and REM stages, cycling every 90-120 minutes.
• N3 sleep (Slow-Wave Sleep), with delta waves, is vital for physical restoration.
• REM sleep involves dreaming, muscle atonia, rapid eye movements, and beta waves; crucial for memory consolidation.
• REM periods lengthen progressively through the night's 4-6 sleep cycles.
• Key neurotransmitters: Acetylcholine (↑REM), Serotonin/Norepinephrine (↑Wakefulness, ↓REM), GABA (↑NREM).
• Melatonin regulates circadian rhythms; Stage N3 sleep decreases with age.