Sleep stages
MCAT glossary · MCAT
Definition
NREM 1-3 (progressively deeper, slow waves in 3), REM (rapid eye movement, dreaming, paradoxical sleep, increased brain activity); ~90 min cycles.
Sleep stages in plain English
Sleep isn't one flat state — your brain cycles through stages roughly every 90 minutes all night. First comes NREM (non-REM) in three levels, 1 to 3, each deeper than the last, until stage 3 shows big slow brain waves — this is your deepest, hardest-to-wake, physically restorative sleep. Then comes REM (rapid eye movement), the 'paradoxical' stage where your eyes dart, vivid dreaming happens, and your brain buzzes almost as if you're awake even though your body stays limp. So a night is like riding a repeating elevator: you sink down into deep slow-wave sleep, rise back up into dream-rich REM, and do it again several times.
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