睡眠演变 Evolution of sleep Sleep is ver anient In the eletrenephalgraphi sense e share it ith all the priates and alst all the ther aals and birds: it a extend ba as far as the reptiles There is se evidene that the t tpes f sleep, dreaing and drealess, depend n the life-stle f the anial, and that predatrs are statistiall uh re liel t drea than pre, hih are in turn uh re liel t experiene drealess sleep In drea sleep, the anial is perfull ibilized and rearabl unrespnsive t external stiuli Drealess sleep is uh shaller, and e have all itnessed ats r dgs ing their ears t a sund hen apparentl fast asleep The fat that deep drea sleep is rare ang pra tda sees learl t be a prdut f natural seletin, and it aes sense that tda, hen sleep is highl evlved, the stupid anials are less frequentl ibilized b deep sleep than the sart nes But h shuld the sleep deepl at all? h shuld a state f suh deep ibilizatin ever have evlved? Perhaps ne useful hint abut the riginal funtin f sleep is t be fund in the fat that dlphins and hales and aquati aals in genera see t sleep ver little There is, b and large, n plae t hide in the ean uld it be that, rather than inreasing an anials vulnerabilit, the Universit f Flrida and Ra eddis f Lndn Universit have suggested this t be the ase It is neivable that anials h are t stupid t be quite n their n initiative are, during perids f high ris, ibilized b the iplaable ar f sleep The pint sees partiularl lear fr the ung f predatr anials This is an interesting ntin and prbabl at least partl true