Sleep is a biologically essential oscillatory brain state governed by interconnected neural circuits, endocrine rhythms, immune pathways, and autonomic patterns. For anesthesiologists, sleep physiology is directly relevant because anesthesia modifies the very circuits responsible for REM, NREM, circadian regulation, and arousal.

1. Non–Rapid Eye Movement (NREM) Sleep

NREM sleep consists of stages N1, N2, and N3:

N1 – Light Sleep

• Transition between wakefulness and sleep

• Decline in alpha activity (8–12 Hz)

• Increased theta activity (4–7 Hz)

N2 – Thalamocortical Sensory Gating

• Sleep spindles (11–16 Hz) generated by the thalamic reticular nucleus

• K-complexes representing cortical down-states

• Essential for initial memory consolidation and sensory isolation

N3 – Slow-Wave Sleep (SWS)

• Dominated by delta oscillations (0.5–4 Hz)

• Maximal parasympathetic dominance

• Physiologic functions:

  • Growth hormone release

  • Immune recalibration

  • Synaptic downscaling

  • Glymphatic clearance of metabolic waste (β-amyloid)

  
  

2. Rapid Eye Movement (REM) Sleep

REM is generated by activation of REM-on cholinergic nuclei in the pons.

Features:

• EEG resembles wakefulness

• Muscle atonia via medullary inhibition

• Active limbic system

• Autonomic variability (tachycardia, arrhythmias, BP swings)

Physiologic roles:

• Emotional integration

• Synaptic stabilization

• Autonomic recalibration

1. Suprachiasmatic Nucleus (SCN)

• Master circadian clock

• Receives retinal light input

• Controls melatonin secretion, cortisol timing, temperature minimum, and sympathetic tone

2. Melatonin

• Secreted at night via SCN → pineal gland pathway

• Primary marker of circadian phase

• Enhances sleep onset and REM sleep

• Suppressed by hospital lighting

3. Cortisol

• Peaks before awakening

• High postoperative cortisol disrupts sleep by stimulating arousal circuits

Homeostatic sleep pressure increases due to:

• Adenosine accumulation

• Activity-dependent metabolic changes

• Neuroinflammation

Key principle: anesthesia does not discharge sleep pressure, hence postoperative recovery may begin with a physiologic “sleep debt.”

References

1. Pace-Schott EF, Hobson JA. The neurobiology of sleep: genetics, cellular physiology and subcortical networks. Nat Rev Neurosci. 2002;3(8):591–605.

2. Brown EN, Lydic R, Schiff ND. General anesthesia, sleep, and coma. N Engl J Med. 2010;363(27):2638–50.

3. Xie L, Kang H, Xu Q, et al. Sleep drives metabolite clearance from the adult brain. Science. 2013;342(6156):373–7.

4. Czeisler CA, Klerman EB. Circadian and sleep-dependent regulation of hormone release in humans. Recent Prog Horm Res. 1999;54:97–130.

5. Borbély AA. A two-process model of sleep regulation. Hum Neurobiol.