Normal Sleep Physiology

Normal Sleep Physiology - Basics of Snooze

• Definition: Reversible state of ↓responsiveness & motor activity.
• Functions: Restoration, memory consolidation, energy conservation.
• Studied via: Polysomnography (PSG) - records EEG (brain), EOG (eyes), EMG (muscles).
• Sleep Stages:
  • NREM (Non-REM): 3 stages (N1, N2, N3/SWS - Slow Wave Sleep).
  • REM (Rapid Eye Movement): Paradoxical sleep; dreaming, muscle atonia.
• Sleep Cycle: NREM-REM alternation; ~90-120 min duration; 4-6 cycles/night.
• Key Neurotransmitters:
  • Wake: Orexin, Histamine.
  • Sleep: GABA, Adenosine.

⭐ Sleep spindles and K-complexes are characteristic EEG waveforms of N2 sleep.

Normal Sleep Physiology - Sleep's Building Blocks

• Sleep cycles: ~90-120 min each, 4-6 cycles/night.
• Two main types: NREM (Non-Rapid Eye Movement) & REM (Rapid Eye Movement).
• NREM: ~75-80% of total sleep; REM: ~20-25%.
• 📌 Mnemonic for EEG waves: Bats Drink Blood (Beta [Awake], Alpha [Drowsy], Theta [N1], Spindles/K-complexes [N2], Delta [N3 - Slow Wave Sleep], Beta [REM]).

⭐ N3 sleep (slow-wave sleep) is most prominent in the first third of the night and decreases with age; it's crucial for physical restoration and growth hormone secretion.

• Sleep architecture: NREM (N1 → N2 → N3 → N2) → REM. First REM usually ~90 min after sleep onset.

Normal Sleep Physiology - Brain's Sleep Controls

• Key Sleep-Wake Centers:
  • SCN (Suprachiasmatic Nucleus): Master circadian clock; light-dark sync.
  • Hypothalamus:
    • VLPO (Ventrolateral Preoptic Nucleus): "Sleep switch"; GABA, galanin (inhibitory).
    • TMN (Tuberomammillary Nucleus): Wake; histamine.
    • Lateral Hypothalamus: Wake; orexin/hypocretin.
  • Brainstem (RAS, Pons - LDT/PPT): Reticular Activating System (RAS) for arousal; Pons (ACh) for REM sleep.
  • Thalamus: Sensory gate during sleep; generates sleep spindles.
  • Basal Forebrain: ACh (wake), Adenosine (sleep pressure).

• Neurotransmitters:

Normal Sleep Physiology - Body's Inner Clock

• Circadian Rhythms: Endogenous ~24-hour physiological cycles (e.g., sleep-wake, temperature).
• Master Clock: Suprachiasmatic Nucleus (SCN) in anterior hypothalamus.
  • Receives light input from retina (Retinohypothalamic Tract - RHT).
  • Synchronizes peripheral clocks. Primary zeitgeber: Light.
• Light-Melatonin Regulation:

• Melatonin: Pineal hormone, "hormone of darkness".
  • Secretion ↑ in dark; Dim Light Melatonin Onset (DLMO) 2-3 hrs pre-sleep.
  • Peaks mid-night; bright light (blue) suppresses.

⭐ SCN's intrinsic cycle is ~24.2 hours; light entrains it daily.

Normal Sleep Physiology - Time's Sleepy Toll

High‑Yield Points - ⚡ Biggest Takeaways

• Sleep is regulated by Process S (homeostatic, adenosine) & Process C (circadian, SCN).
• NREM sleep includes N3 (slow-wave sleep, delta waves); REM sleep shows active EEG & muscle atonia.
• Sleep spindles & K-complexes are hallmarks of N2 sleep.
• Melatonin regulates circadian rhythm; adenosine promotes sleep.
• Sleep cycles are 90-120 minutes; N3 predominates early, REM later.
• Ageing leads to ↓ total sleep time & ↓ N3 sleep.