睡眠的演变(Evolution of sleep)

sleepisveryancient.intheelectroencephalographicsenseweshareitwithalltheprimatesandalmostalltheothermammalsandbirds:itmayextendbackasfarasthereptiles.

thereissomeevidencethatthetwotypesofsleep,dreaminganddreamless,dependonthelife-styleoftheanimal,andthatpredatorsarestatisticallymuchmorelikelytodreamthanprey,whichareinturnmuchmorelikelytoexperiencedreamlesssleep.indreamsleep,theanimalispowerfullyimmobilizedandremarkablyunresponsivetoexternalstimuli.dreamlesssleepismuchshallower,andwehaveallwitnessedcatsordogscockingtheirearstoasoundwhenapparentlyfastasleep.thefactthatdeepdreamsleepisrareamongpraytodayseemsclearlytobeaproductofnaturalselection,anditmakessensethattoday,whensleepishighlyevolved,thestupidanimalsarelessfrequentlyimmobilizedbydeepsleepthanthesmartones.butwhyshouldtheysleepdeeplyatall?whyshouldastateofsuchdeepimmobilizationeverhaveevolved?

perhapsoneusefulhintabouttheoriginalfunctionofsleepistobefoundinthefactthatdolphinsandwhalesandaquaticmammalsingeneraseemtosleepverylittle.thereis,byandlarge,noplacetohideintheocean.coulditbethat,ratherthanincreasingananimalsvulnerability,theuniversityoffloridaandraymeddisoflondonuniversityhavesuggestedthistobethecase.itisconceivablethatanimalswhoaretoostupidtobequiteontheirowninitiativeare,duringperiodsofhighrisk,immobilizedbytheimplacablearmofsleep.thepointseemsparticularlyclearfortheyoungofpredatoryanimals.thisisaninterestingnotionandprobablyatleastpartlytrue.