Normal Sleep and Sleep-Wake Regulation
Architecture and the two-process model
Normal sleep is organized into N1, N2, N3 (slow-wave sleep) and REM, cycling roughly every 90 minutes, with N3 front-loaded and REM back-loaded across the night. Sleep-onset latency under 30 minutes, sleep efficiency above 85%, and a normal adult total sleep time of 7-9 hours frame the healthy baseline.
Understand the two-process model: Process S (homeostatic sleep drive rising with wakefulness, dissipating with sleep) and Process C (circadian rhythm paced by the suprachiasmatic nucleus). Light is the dominant zeitgeber. Melatonin onset occurs in dim light roughly 2 hours before habitual sleep (dim light melatonin onset, DLMO).
Traps to watch
- REM vs NREM physiology: REM has muscle atonia with EEG activation; NREM N3 has high-voltage slow waves. Questions often swap these.
- Chronotype and age shifts: Adolescents phase-delay; older adults phase-advance and lose N3.
- Arousal threshold: Lowest in REM for some stimuli; CO2 arousal responses are blunted in REM.
- Sleep spindle and K-complex are N2 hallmarks, not N3.
Rule of thumb
When a stem describes a patient whose sleep timing drifts later with preserved sleep quality and duration, think circadian phase delay, not insomnia. When sleep drive is intact but timing is wrong, the problem is Process C.
Worked example
A 16-year-old cannot fall asleep until 1 a.m. and sleeps until 9 a.m. on weekends, feeling fine otherwise. Total sleep time is normal; the issue is delayed circadian phase. Treatment is morning bright light and gradual advance, not hypnotics.