Natural Patterns of Sleep - Healthy Sleep - Harvard University

The average length of the first NREM-REM sleep cycle is between 70 and 100 minutes; the average length of the second and later cycles is about 90 to 120 minutes ... HealthSleep WhySleepMattersTheScienceofSleepWhatisSleep?TheCharacteristicsofSleepNaturalPatternsofSleepSleepLabHowisSleepRegulated?VariationsinSleepGettingtheSleepYouNeed Search NaturalPatternsofSleep AtaGlance Sleepwasonceconsideredaninactive,orpassive,stateinwhichboththebodyandthebrain"turnedoff"torestandrecuperatefromtheday'swakingactivities.Scientistshavesincefoundthatthebraingoesthroughcharacteristicpatternsofactivitythroughouteachperiodofsleep,andthatitissometimesmoreactivewhenwe'reasleepthanwhenwe'reawake.Understandingthesepatterns,andthefactorsthataffectthem,mayhelpinmakingchoicesthatwillleadtobetterqualitysleep. Featuresonthispage REMandNREMSleep SleepPatternsAcrosstheNight WakinguptoSleepOurbodiesrequiresleepinordertomaintainproperfunctionandhealth.Infact,weareprogrammedtosleepeachnightasameansofrestoringourbodiesandminds.Twointeractingsystems—theinternalbiologicalclockandthesleep-wakehomeostat—largelydeterminethetimingofourtransitionsfromwakefulnesstosleepandviceversa.Thesetwofactorsalsoexplainwhy,undernormalconditions,wetypicallystayawakeduringthedayandsleepatnight.Butwhatexactlyhappenswhenwedriftofftosleep?Priortotheeraofmodernsleepresearchintheearly1920s,scientistsregardedsleepasaninactivebrainstate.Itwasgenerallyacceptedthatasnightfellandsensoryinputsfromtheenvironmentdiminished,sotoodidbrainfunction.Inessence,scientiststhoughtthatthebrainsimplyshutdownduringsleep,onlytorestartagainwhenmorningcame. EEGsareusedinsleepstudiestomonitorbrainactivityduringvariousstagesofsleep. In1929,aninventionthatenabledscientiststorecordbrainactivitychallengedthiswayofthinking.Fromrecordingsknownaselectroencephalograms(EEGs),researcherscouldseethatsleepwasadynamicbehavior,oneinwhichthebrainwashighlyactiveattimes,andnotturnedoffatall.Overtime,sleepstudiesusingEEGsandotherinstrumentsthatmeasuredeyemovementsandmuscleactivitywouldrevealtwomaintypesofsleep.Theseweredefinedbycharacteristicelectricalpatternsinasleepingperson'sbrain,aswellasthepresenceorabsenceofeyemovements. AnEEGoftypicalREMsleep. Thetwomaintypesofsleeparerapid-eye-movement(REM)sleepandnon-rapid-eye-movement(NREM)sleep.OnanEEG,REMsleep,oftencalled"activesleep,"isidentifiablebyitscharacteristiclow-amplitude(small),high-frequency(fast)wavesandalpharhythm,aswellastheeyemovementsforwhichitisnamed.Manysleepexpertsthinkthattheseeyemovementsareinsomewayrelatedtodreams.Typically,whenpeopleareawakenedfromREMsleep,theyreportthattheyhadbeendreaming,oftenextremelyvividandsometimesbizarredreams.Incontrast,peoplereportdreamingfarlessfrequentlywhenawakenedfromNREMsleep.Interestingly,duringREMsleepmusclesinthearmsandlegsaretemporarilyparalyzed.Thisisthoughttobeaneurologicalbarrierthatpreventsusfrom"actingout"ourdreams.NREMsleepcanbebrokendownintothreedistinctstages:N1,N2,andN3.IntheprogressionfromstageN1toN3,brainwavesbecomeslowerandmoresynchronized,andtheeyesremainstill.InstageN3,thedeepeststageofNREM,EEGsrevealhigh-amplitude(large),low-frequency(slow)wavesandspindles.Thisstageisreferredtoas"deep"or"slow-wave"sleep. REMandNREMSleep (1:21)Dr.ThomasScammelldescribesthedifferentstagesofsleepandtheircharacteristics. watchvideo CyclingatNight EEGrecordingsshowingallthreestagesoftypicalNREMsleep. Inhealthyadults,sleeptypicallybeginswithNREMsleep.ThepatternofclearrhythmicalphaactivityassociatedwithwakefulnessgiveswaytoN1,thefirststageofsleep,whichisdefinedbyalow-voltage,mixed-frequencypattern.ThetransitionfromwakefulnesstoN1occurssecondstominutesafterthestartofthesloweyemovementsseenwhenapersonfirstbeginstonodoff.ThisfirstperiodofN1typicallylastsjustonetosevenminutes.Thesecondstage,orN2,whichissignaledbysleepspindlesand/orKcomplexesintheEEGrecording,comesnextandgenerallylasts10to25minutes.AsN2sleepprogresses,thereisagradualappearanceofthehigh-voltage,slow-waveactivitycharacteristicofN3,thethirdstageofNREMsleep.Thisstage,whichgenerallylasts20to40minutes,isreferredtoas"slow-wave,""delta,"or"deep"sleep.AsNREMsleepprogresses,thebrainbecomeslessresponsivetoexternalstimuli,anditbecomesincreasinglydifficulttoawakenanindividualfromsleep. SleepPatternsAcrosstheNight (0:46)Dr.ThomasScammelldescribesthecyclesofREMandNREMsleepthatoccurthroughoutthenight. watchvideo FollowingtheN3stageofsleep,aseriesofbodymovementsusuallysignalsan"ascent"tolighterNREMsleepstages.Typically,a5-to10-minuteperiodofN2precedestheinitialREMsleepepisode.REMsleepcomprisesabout20to25percentoftotalsleepintypicalhealthyadults.NREMsleepandREMsleepcontinuetoalternatethroughthenightinacyclicalfashion.Mostslow-waveNREMsleepoccursinthefirstpartofthenight;REMsleepepisodes,thefirstofwhichmaylastonlyonetofiveminutes,generallybecomelongerthroughthenight.Duringatypicalnight,N3sleepoccupieslesstimeinthesecondcyclethanthefirstandmaydisappearaltogetherfromlatercycles.TheaveragelengthofthefirstNREM-REMsleepcycleisbetween70and100minutes;theaveragelengthofthesecondandlatercyclesisabout90to120minutes.ThereasonforsuchaspecificcyclingpatternofNREMandREMsleepacrossthenightisunknown.SomescientistsspeculatethatspecificsequencesofNREMandREMsleepoptimizebothphysicalandmentalrecuperationaswellassomeaspectsofmemoryconsolidationthatoccurduringsleep,butthishasnotbeenconfirmed.ShiftingSleepPatterns ThishypnogramshowsthetypicalpatternsofREMandNREMsleepthroughoutthenight. Sleeppatternscanbeaffectedbymanyfactors,includingage,theamountofrecentsleeporwakefulness,thetimeofthedayornightrelativetoanindividual’sinternalclock,otherbehaviorspriortosleepsuchasexercise,stress,environmentalconditionssuchastemperatureandlight,andvariouschemicals.Forexample,forthefirstyearoflife,sleepoftenbeginsintheREMstate.ThecyclicalalternationofNREM-REMsleepinnewbornsispresentfrombirthbutat50to60minutesismuchshorterthanthe90-minutecyclesthatoccurinadults.ConsolidatednocturnalsleepandfullydevelopedEEGpatternsoftheNREMsleepstagesemergeonlyaftertwotosixmonths.Slow-wavesleepisgreatestinyoungchildrenanditdecreasessteadilywithage,evenifsleepdurationdoesnotchange.Thismayberelatedtochangesinthestructureandfunctionofthebrain.Sleephistory—thequantityandqualityofanindividual’ssleepinrecentdays—canalsohavedramaticeffectsonsleeppatterns.Repeatedlymissinganight’ssleep,anirregularsleepschedule,orfrequentdisturbanceofsleepcanresultinaredistributionofsleepstages,forinstance,prolongedanddeeperperiodsofslow-waveNREMsleep.Drugsmayaffectsleepstagesaswell.Forexample,alcoholbeforesleeptendstosuppressREMsleepearlyinthenight.Asthealcoholismetabolizedlaterinthenight,REMsleeprebounds.However,awakeningsalsobecomemorefrequentduringthistime.Tolearnmoreaboutthemanyfactorsthataffectsleeppatterns,seeExternalFactorsThatInfluenceSleep.DaytimeNapping Daytimenappinghelpstorestoreone’senergyandalertness. Althoughitiscommonforpeopleinmanywesternsocietiestosleepinasingleconsolidatedblockofabouteighthoursduringthenight,thisisbynomeanstheonlysleeppattern.Infact,followingthisscheduleandforegoinganafternoonnapwouldseemhighlyabnormaltomanypeoplearoundtheworld.Inmanycultures,particularlythosewithrootsintropicalregions,afternoonnappingiscommonplaceandisbuiltintodailyroutines.Andalthoughtheexacttimingofnapsisnotofficiallyscheduled,itisnotuncommonforstoresandgovernmentofficestocloseandformanyactivitiestostopforanhourortwoeveryafternoon.Afternoonnaptimetypicallycoincideswithabrieflaginthebody'sinternalalertingsignal.Thissignal,whichincreasesthroughoutthedaytooffsetthebody'sincreasingdrivetosleep,wanesslightlyinmid-afternoon,givingsleepdriveaslightedge.Nappingalsotypicallyhappensduringthewarmestperiodofthedayandgenerallyfollowsalargemid-daymeal,whichexplainswhyafternoonsleepinessissooftenassociatedwithwarmafternoonsunandheavylunches.BookshelfAserinskyE,KleitmanN.1953.Regularlyoccurringperiodsofeyemotility,andconcomitantphenomena,duringsleep.Science.118:273-274.IberAncoli-IsraelS,ChessonAL,QuanSF.2007.TheAASMManualfortheScoringofSleepandAssociatedEvents:Rules,TerminologyandTechnicalSpecification.AmericanAcademyofSleepMedicine:Westchester,IL.Afternoonnapsformostpeopletypicallylastbetween30and60minutes.Anylongerandthereisariskoffallingintodeepsleepandhavingadifficulttimewaking.Followinganap,havingdissipatedsomeoftheaccumulatedsleepdrive,manypeoplereportfeelingbetterabletostayawakeandalertinthelateafternoonandevening.Thisincreasedalertnesstypicallycausespeopletogotobedlaterandgenerallytosleeplessatnightthanpeoplewhodonottakenaps.Accordingtosleepexperts,nappingcanbeagoodwayforpeoplewhodonotsleepwellatnighttocatchup.Theydocaution,however,thatpeoplewithinsomniamaymaketheirnighttimesleepproblemworsebysleepingduringtheday.Otherwise,theygenerallyrecommendnapsforpeoplewhofeeltheybenefitfromthem.Tolearnmoreaboutotherpatternsassociatedwithsleep,gototheSleepLab,TheCharacteristicsofSleep,andChangesinSleepwithAge. backtotop ThiscontentwaslastreviewedonDecember18,2007 HarvardUniversity AresourcefromtheDivisionofSleepMedicineatHarvardMedicalSchool ProducedinpartnershipwithWGBHEducationalFoundation AboutthisSite|SiteMap|Glossary|VideoIndex|TechnicalHelp Feedback|Credits|Disclosures|Disclaimers|UnderstandingSleep