REM sleep is the stage of sleep characterized by rapid eye movements, vivid dreaming, near-complete muscle atonia, and brain-wave activity that resembles wakefulness. It occupies roughly one-fifth of a normal night’s sleep in healthy adults.

Periods of REM sleep recur several times during the night, growing longer toward morning. The eyes dart beneath closed lids; heart rate and breathing become irregular; the body is effectively paralyzed, preventing the enactment of dreams. Most narrative dreaming is reported from awakenings in this stage. REM sleep appears to support memory consolidation, emotional processing, and aspects of brain development.

Deprivation of REM sleep leads to a rebound in which the proportion of REM increases on subsequent nights. Certain antidepressants suppress REM. Disorders such as REM-sleep behavior disorder allow the physical acting-out of dreams because the normal paralysis fails. The stage remains one of the most distinctive and intensively studied features of mammalian sleep.

When the eyes move rapidly and the mind fills with dreams while the body stays still, the sleeper is in REM.