REM Sleep

REM sleep, in full rapid eye movement sleep, one of two phases in the sleep cycle, in which a person experiences dreams, atonia (reduced muscle tone), irregular closed eye movements, and elevated levels of brain activity. The other phase of the human sleep cycle is known as non-rapid eye movement (NREM) sleep. A single sleep cycle, with both NREM and REM phases, lasts about 90 to 110 minutes; most adults complete the cycle 4 to 6 times per night. In addition to humans, REM sleep has been identified in other mammals, in birds, and in a few species of reptiles and invertebrates; its role in the sleep cycle varies between species.

About 20 to 25 percent of a human adult’s sleep cycle and more than 50 percent of an infant’s sleep cycle consists of REM sleep. In adults, early REM periods are shorter (about 10 minutes), with their duration increasing up to about 60 minutes in adults as the night progresses. During REM sleep, the brain selectively suppresses responses to sensory stimuli; for example, while loud noises may not awaken an exhausted mother, the sound of her infant crying may cause her to awaken immediately.