REM sleep (rapid eye movement sleep) is one of the two main phases of sleep, alongside non-REM sleep. During REM, your eyes twitch behind closed lids, brain activity looks a lot like wakefulness, and dreaming usually happens. Your skeletal muscles normally go limp so you do not act out dreams.
Sleep cycles through REM and non-REM about every 80 to 100 minutes, typically four to six times a night. You usually get more REM later in the night. Newborns spend more time in REM than adults do.