Dream sleep
REM sleep: the stage of sleep with rapid eye movements, a busy brain and the most vivid dreams, while the body's muscles go slack. It comes back about every 90 minutes, in longer spells towards morning.
Dream sleep is usually called REM sleep, after the rapid eye movements that mark it. It's one of the two main kinds of sleep; the other, non-REM, covers light and deep sleep. Evidence: solid It was first described in 1953. It comes back about every 90 minutes, the spells get longer towards morning, and in adults it adds up to roughly a fifth to a quarter of the night. Evidence: solid During it, brain waves look much like waking ones, and almost every muscle goes slack except the ones for breathing and moving the eyes. Evidence: solid People woken from it often recall dreams. Evidence: solid
Where it gets oversold: that it's the only time you dream. People woken from other stages of sleep report dreams too. Evidence: decent What it's for isn't settled either. Helping memory is one leading idea, not a proven job. Evidence: decent And your tracker's dream-sleep number is a rough guess: tested against a sleep lab, seven consumer devices picked up sleep well, but their sleep stages were inconsistent. Evidence: decent
Why it matters
Alcohol pushes it later and cuts it. That's the core of alcohol is not a sleep aid. Evidence: solid One review found the cut starting from about two drinks and growing with each step up. Evidence: decent In a lab study of seven young men, a hot, humid room (35°C, 75% humidity) meant less of it, one reason to cool the room. Evidence: decent It's one of the stages the brain may use for consolidation, though which stage does what is still argued over. Evidence: decent