REM sleep, which stands for Rapid Eye Movement sleep, is a distinct and essential stage of the sleep cycle characterized by rapid, random eye movements, heightened brain activity similar to wakefulness, and temporary paralysis of the voluntary muscles (known as REM atonia). It is the stage of sleep most strongly associated with vivid, narrative dreaming. REM sleep was first described in the 1950s by researchers who noticed that the eyes of sleeping subjects moved rapidly in various directions during certain periods of sleep. This discovery fundamentally changed our understanding of sleep as an active, rather than passive, brain state. In a typical night, REM sleep alternates with non-REM sleep in cycles of approximately 90 minutes. The first REM period is relatively short, lasting only a few minutes, but REM periods become progressively longer toward morning, meaning that the majority of REM sleep occurs in the second half of the night. Adults spend approximately 20 to 25 percent of their total sleep time in REM sleep. REM sleep plays several important and distinct roles. It is closely linked to emotional processing — during REM sleep, the brain processes and integrates emotional experiences, which may explain why REM sleep deprivation tends to increase emotional reactivity and instability. REM sleep is also critical for memory consolidation, particularly for procedural skills, creative problem-solving, and the integration of newly learned information. REM sleep can be suppressed by alcohol, certain antidepressants, and sleep deprivation. A rebound effect — in which REM sleep is dramatically increased after a period of REM suppression — is well documented. Conditions such as narcolepsy and REM sleep behavior disorder directly involve dysfunction of REM sleep regulation.