Rapid eye movement (REM) sleep

Rapid eye movement sleep is the sleep stage marked by darting eye movements, vivid dreaming, brain activity resembling wakefulness and near-total muscle paralysis.

It first appears about ninety minutes after falling asleep and recurs in longer episodes through the night, making up roughly a fifth to a quarter of adult sleep. During REM, breathing and heart rate become irregular, body temperature regulation loosens, and the brain appears to consolidate memories and process emotions. The temporary paralysis keeps people from acting out dreams; when it fails, the result is REM sleep behavior disorder.

Sleep laboratories identify REM by combining electroencephalogram, eye movement and muscle tone recordings during polysomnography. REM percentage and the time it takes to reach REM are reported in sleep studies and used as outcomes in research. Cannabinoids have been observed to alter REM measures in some studies, and such changes are not the same as a clinical benefit.