Definition

The phase of sleep in which most dreams occur. During REM sleep, a person’s brain activity, breathing, heart rate, and blood pressure increase, and the eyes move rapidly while closed. The muscles in the arms and legs become temporarily unable to move. REM sleep is thought to play an important role in memory and learning. During normal sleep, a person goes through four to five sleep cycles that last about 90 minutes each and include both REM sleep and non-REM sleep (light to deep sleep). Also called rapid eye movement sleep.