Normal Sleep and Sleep-Wake Regulation
Core physiology
Sleep is organized into NREM (N1, N2, N3) and REM, cycling roughly every 90 minutes, with N3 front-loaded in the night and REM back-loaded toward morning. N1 is a transitional state with slow eye movements and attenuation of alpha; N2 is defined by sleep spindles and K-complexes; N3 requires at least 20% slow-wave (0.5-2 Hz, >75 microvolt) activity; REM shows low-voltage mixed-frequency EEG, rapid eye movements, and skeletal muscle atonia with preserved phasic twitches. Normal sleep efficiency is generally 85% or higher, with sleep-onset latency under 30 minutes.
Regulatory models
Understand the two-process model: Process S (homeostatic sleep pressure, tracked by adenosine accumulation and reflected in slow-wave activity) and Process C (circadian timing generated by the suprachiasmatic nucleus). Sleep propensity is maximal when Process S is high and the circadian alerting signal is low. Melatonin secretion rises in dim light and is suppressed by bright light; core body temperature nadirs in the early morning. Also know normal developmental and aging changes: total sleep time declines across the lifespan, slow-wave sleep falls sharply with age, and circadian phase advances in older adults.
Question traps
Questions love to blur the boundaries: K-complexes alone do not define N3; REM atonia can be lost in REM sleep behavior disorder; and a brief arousal does not necessarily change the stage. Watch for distractors that attribute circadian control to adenosine (it is homeostatic) or that place maximal REM in the first cycle (it is later cycles).
Rule of thumb
First third of the night = N3; last third = REM. If a stem asks where slow-wave sleep concentrates, answer early; if it asks where REM concentrates, answer late.