Periods of rem sleep alternate with periods of non-rem sleep in a cycle that recurs every minutes or so

REM sleep is a period of sleep where your eyes move rapidly (Rapid Eye Movement). After a period of REM sleep, your body experiences 5 to 15 minute periods of non-REM sleep. During this lighter sleep it is much easier for you to be awakened. There are different periods of REM sleep which can last from 10 minutes to one hour. The longer you sleep, the longer the REM period will last. This cycle will occur on average every 90 minutes.