Whycansomepeoplesleepthroughnoiseslikeahonkingcarorflushingtoiletwhileothersareawakenedbythelightestsound?
TofindtheanswersleepresearchersatMassachusettsGeneralHospitalconductedanunusualstudyof12self-describeddeepsleepers.Aftertestsconfirmedthatthehealthyvolunteersweresolidsleeperstheytookpartinathree-nightstudyintheuniversity’ssleeplaboratory.Theparticipantsspentthenightinabigandcomfortableroom.Buttheroomalsoincludedfourspeakerspositionednearthetopofthebed.
Duringthenightthedeepsleepersweresubjectedto14differentrecordedsoundslikestreettraffictoiletsflushingandanairplaneflyingoverhead.Nextdoortheresearchersmonitoredtheirsleeppatternsandbrainwaves.
Asexpectedalloftheparticipantssleptrelativelywellbutthereweredifferencesinhowtheyrespondedtothenoisyinterruptions.Someofthesleepersdidn’twakeupevenwhenasoundwasblastedat70decibels(分贝);otherswereawakenedbysoundsat40or50decibels.
Theresearchersdiscoveredthatthedifferenceinasleeper’sreactiontonoisecouldbepredictedbythelevelofbrainactivitycalled“sleepspindles(纺锤体)”.Asleepspindleisaburstofhigh-frequencybrainactivitycomingfromdeepinsidethebrainduringsleep.Thesourceofthespindlesisthethalamus(丘脑)apartofthebrainthatsendssensoryinformationtotherestofthecortex(皮层).
BeforethestudytheMassachusettsresearcherstheorizedthatthespindlesarethebrain’swayofpreventingsensoryinformationfrompassingthroughthethalamusandwakingtherestofthebrainduringsleep.Theyfoundthatsleeperswhoexperiencedthemostsleepspindlesduringthenightwerealsothesoundestsleepersandwereleastlikelytobeawakenedbynoise.
Scientistsalreadyknowthatmostpeoplebecomelightersleeperswithagemostlikelybecauseolderpeopleexperienceless“slowwavesleep”whichisthedeepeststageofsleep.Peoplealsoproducefewersleepspindlesastheyage.Butevenwhencontrollingforthestageofsleepapersonwasinthenumberofsleepspindlesstillpredictedtheirriskforawakeningbecauseofnoise.
Moreresearchisneededbutthefindingssuggestthatabetterunderstandingofsleepspindles