What Is REM Sleep?

REM (rapid eye movement) sleep is a distinct stage of sleep most closely associated with vivid, memorable dreaming, though it isn’t the only stage in which dreaming occurs.

Discovery

REM sleep was first identified in 1953 by researchers Eugene Aserinsky and Nathaniel Kleitman at the University of Chicago, who noticed periods of rapid eye movement accompanied by wake-like brain activity in otherwise sleeping research subjects. The discovery essentially founded modern sleep science as a distinct research field, and REM sleep remains one of its central subjects of study.

What Happens During REM Sleep

REM sleep is marked by rapid eye movements, near-total paralysis of the body’s voluntary muscles (with the notable exceptions of the eye muscles and the diaphragm), brain activity in some regions that closely resembles wakefulness, and irregular heart rate and breathing. This combination of an active brain and a paralysed body is thought to be why REM sleep produces such vivid dream content while, under normal circumstances, preventing the sleeper from physically acting it out.

Where REM Fits in the Sleep Cycle

REM sleep alternates with non-REM (NREM) sleep stages throughout the night in cycles lasting roughly ninety minutes. Early REM periods tend to be short - around ten minutes - and lengthen across the night, with the longest REM periods typically occurring shortly before natural waking. See How Long Do Dreams Last? for more on how this relates to dream length.

A Common Oversimplification

REM sleep and dreaming are closely linked, but they aren’t identical. Dreaming - generally less vivid, less narrative, and less often recalled - also occurs during NREM sleep. See Do We Dream During NREM Sleep? for that distinction, and Why Do We Dream? for the broader open question of what dreaming is for.