演变睡眠(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.