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1、大脑是一个解释性的器官AboutthreeyearsagoEvaSalemgotintosometroublewithacrocodile.Itsnappedherhandinitsjaws.Inapanic,shemanagedtoknockoutthecrocodileandfreeherself.Then,shewokeup."Iimaginethat'swhatit'slikewhenyou'reonheroin.That'swhatmydreamswerelike—vivid,crazyandactive," shesays.Salem,a
2、newmother,hadbeenbreast-feedingherdaughterforfivemonthsbeforethecroc-attackdream,livingonfourhoursofsleepanight.Ifshedidsleepafullnight,herdreamsboomeranged,becomingsovividthatshefeltlikeshewasn'tsleepingatall.Dreamsareamazinglypersistent.Missafewfromlackofsleepandthebrainkeeps
3、score,forcingpaybacksoonaftereyelidsclose."Nature'ssoftnurse," asShakespearecalledsleep,isn'tsosoftafterall."Whensomeoneissleepdeprivedweseegreatersleepintensity,meaninggreaterbrainactivityduringsleep; dreamingisdefinitelyincreasedandlikelymorevivid," saysneurologistMarkMahowal
4、doftheUniversityofMinnesotaanddirectoroftheMinnesotaRegionalSleepDisordersCenterinMinneapolis.ThephenomenoniscalledREMrebound.REMrefersto"rapideyemovement," thedartingoftheeyesunderclosedlids.Inthisstatewedreamthemostandourbrainactivityeerilyresemblesthatofwakinglife.Yet,atthes
5、ametime,ourmusclesgoslackandwelieparalyzed—atoemightwiggle,butessentiallywecan'tmove,asifourbrainisprotectingourbodiesfromactingoutthestorieswedream.SleepisdividedintoREMandfourstagesofnon-REM; eachhasadistinctbrainwavefrequency.Stageoneofnon-REMisthenoddingoffperiodwhereoneisb
6、etweensleepingandwaking; it'ssometimespunctuatedwithasensationoffallingintoahole.Instagetwothebrainslowswithonlyafewburstsofactivity.Thenthebrainpracticallyshutsoffinstagesthreeandfourandshiftsintoslow-wavesleep,whereheartandbreathingratesdropdramatically.Onlyafter70minutesofno
7、n-REMsleepdoweexperienceourfirstperiodofREM,anditlastsonlyfiveminutes.Atotalnon-REM–REMcycleis90minutes; thispatternrepeatsaboutfivetimesoverthecourseofanight.Asthenightprogresses,however,non-REMstagesshortenandtheREMperiodsgrow,givingusa40-minutedreamscapejustbeforewaking.Theo
8、nlywayscientistscanstudyREMdeprivationisbytorturoussle
