Dreamsaresuccessionsofimages,ideas,emotions,andsensationsthatoccurinvoluntarilyinthemindduringcertainstagesofsleep.Thecontentandpurposeofdreamsarenotdefinitivelyunderstood,thoughtheyhavebeenatopicofscientificspeculation,aswellasasubjectofphilosophicalandreligiousinterest,throughoutrecordedhistory.Thescientificstudyofdreamsiscalledoneirology.
Dreamsmainlyoccurintherapid-eyemovement(REM)stageofsleep—whenbrainactivityishighandresemblesthatofbeingawake.REMsleepisrevealedbycontinuousmovementsoftheeyesduringsleep.Attimes,dreamsmayoccurduringotherstagesofsleep.However,thesedreamstendtobemuchlessvividormemorable.
Dreamscanlastforafewseconds,oraslongastwentyminutes.PeoplearemorelikelytorememberthedreamiftheyareawakenedduringtheREMphase.Theaveragepersonhasabout3to5dreamspernight,butsomemayhaveupto7dreamsinonenight.Thedreamstendtolastlongerasthenightprogresses.Duringafull8-hournightsleep,twohoursofitisspentdreaming.