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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,anewmother,ha
2、dbeenbreast-feedingherdaughterforfivemonthsbeforethecroc-attackdream,livingonfourhoursofsleepanight.Ifshedidsleepafullnight,herdreamsboomeranged,becomingsovividthatshefeltlikeshewasn'tsleepingatall.Dreamsareamazinglypersistent.Missafewfromlackofsleepandthebrainkeepsscore,forcingpaybacksoon
3、aftereyelidsclose."Nature'ssoftnurse," asShakespearecalledsleep,isn'tsosoftafterall."Whensomeoneissleepdeprivedweseegreatersleepintensity,meaninggreaterbrainactivityduringsleep; dreamingisdefinitelyincreasedandlikelymorevivid," saysneurologistMarkMahowaldoftheUniversityofMinnesotaanddirect
4、oroftheMinnesotaRegionalSleepDisordersCenterinMinneapolis.ThephenomenoniscalledREMrebound.REMrefersto"rapideyemovement," thedartingoftheeyesunderclosedlids.Inthisstatewedreamthemostandourbrainactivityeerilyresemblesthatofwakinglife.Yet,atthesametime,ourmusclesgoslackandwelieparalyzed—atoem
5、ightwiggle,butessentiallywecan'tmove,asifourbrainisprotectingourbodiesfromactingoutthestorieswedream.SleepisdividedintoREMandfourstagesofnon-REM; eachhasadistinctbrainwavefrequency.Stageoneofnon-REMisthenoddingoffperiodwhereoneisbetweensleepingandwaking; it'ssometimespunctuatedwithasensati
6、onoffallingintoahole.Instagetwothebrainslowswithonlyafewburstsofactivity.Thenthebrainpracticallyshutsoffinstagesthreeandfourandshiftsintoslow-wavesleep,whereheartandbreathingratesdropdramatically.Onlyafter70minutesofnon-REMsleepdoweexperienceourfirstperiodofREM,anditlastsonlyfiveminutes.At
7、otalnon-REM–REMcycleis90minutes; thispatternrepeatsaboutfivetimesoverthecourseofanight.Asthenightprogresses,however,non-REMstagesshortenandtheREMperiodsgrow,givingusa40-minutedreamscapejustbeforewaking.TheonlywayscientistscanstudyREMdeprivationisbytorturoussle
