Rapid eye movement (REM) sleep is one phase of the sleep cycle. During REM sleep, your breathing speeds up, your eyes move, your voluntary muscles are immobile, and you experience dreams. REM sleep is believed to play a role in memory storage and learning, perhaps by clearing out irrelevant information from the mind.
About every 90 minutes, or three to five times a night, you enter REM sleep. While the first episode usually lasts for only a few minutes, REM time increases over the course of the night, with the final episode lasting up to 30 minutes. Once REM sleep ends, you cycle back through three stages of non-REM sleep.