REM sleep is a paradoxical state — brain activity resembles waking, but skeletal muscles are paralyzed. Occurs in 4-6 cycles per night, with progressively longer episodes toward morning.
Mechanism — Emotional memory consolidation, procedural learning, threat-rehearsal, and creative problem-solving cluster in REM. Acetylcholine peaks; serotonin and norepinephrine drop to near zero. Vivid dreaming is REM dominant.
Use case — Most adults need 90-120 minutes of REM per night. Optimize by avoiding alcohol (suppresses REM), THC (suppresses REM), late-day intense exercise. Cool bedroom, dark room, consistent wake time.
Caveats — Consumer sleep trackers underestimate REM. Withdrawal-rebound (after stopping alcohol, THC, or SSRIs) often produces vivid 'too much REM' nights — normalizes within 1-2 weeks.