REM

Meaning: REM is one stage of the roughly 90-minute sleep cycle, alongside NREM stages N1, N2 and N3 (slow-wave sleep). Early in the night, N3 takes up more of each cycle; REM periods get longer toward morning. In REM the EEG shows fast, low-amplitude, desynchronized waves like those of an alert waking brain. The eyes dart beneath closed lids, heart rate and breathing become irregular, and brainstem circuits inhibit motor neurons, producing muscle atonia. Brain active but body paralyzed is why REM is called paradoxical sleep. The activation-synthesis hypothesis proposes that the cortex builds a story from random brainstem activity during REM.

On the MCAT: A passage may show EEG and muscle-tone (EMG) tracings or a graph of sleep stages across a night. Identify REM as wake-like EEG with flat EMG. Predict that people woken from REM often report vivid dreams, that REM rebound (extra REM) follows REM deprivation, and that infants spend a larger share of sleep in REM than older adults do.

Common confusion: Students equate REM with deepest sleep and say dreaming occurs only in REM. Deepest sleep, judged by how hard it is to wake someone, is N3. Dreams also occur in NREM, usually shorter and less vivid. Sleepwalking arises from N3, when muscle tone is still present, whereas REM atonia normally prevents acting out dreams.