thr is som vidnc that th two typs of slp, draming and dramlss, dpnd on th lif-styl of th animal, and that prdators ar statistically much mor likly to dram than pry, which ar in turn much mor likly to xprinc dramlss slp. in dram slp, th animal is powrfully immobilizd and rmarkably unrsponsiv to xtrnal stimuli.dramlss slp is much shallowr, and w hav all witnssd cats or dogs cocking thir ars to a sound whn apparntly fast aslp. th fact that dp dram slp is rar among pry today sms clarly to b a product of natural slction, and it maks sns that today, whn slp is highly volvd, th stupid animals ar lss frquntly immobilizd by dp slp than th smart ons. but why should thy slp dply at all? why should a stat of such dp immobilization vr hav volvd? prhaps on usful hint about th original function of slp is to b found in th fact that dolphins and whals and aquatic mammals in gnral sm to slp vry littl. thr is, by and larg, no plac to hid in th ocan. could it b that, rathr than incrasing an animal's vulnrability, th function of slp is to dcras it? wils wbb of th univrsity of florida and ray mddis of london univrsity hav suggstd this to b th cas. it is concivabl that animals who ar too stupid to b quit on thir own initiativ ar, during priods of high risk, immobilizd by th implacabl arm of slp. th point sms particularly clar for th young of prdatory animals. this is an intrsting notion and probably at last partly tru.睡眠的进化睡眠是古老的。 从脑电图上看，我们人类和所有灵长目动物以及几乎所有的哺乳动物和鸟类都一样需要睡眠;甚至爬行类动物也有睡眠。有证据显示，有梦睡眠和无梦睡眠这两种类型的睡眠取决于该动物的生活方式。 从统计上看，食肉动物比被捕食动物有更多的有梦睡眠，而被捕食动物更多地无梦睡眠。 动物在有梦睡眠时，被有效地解除动作能力，并且对外界刺激缺乏反应。 无梦睡眠则要浅得多。 我们都看到过猫和狗在显然的酣睡中，有一点响动耳朵就会竖起来。 被捕食动物很少有深度的有梦睡眠，这看来显然是自然选择的结果。 而且这一点是有道理的：当睡眠高度进化以后，愚笨的动物比聪明的动物更少在深度睡眠状态下丧失动作能力。 但是动物为什么要进入深度睡眠呢?为什么这样的无动作状态也会进化出来呢? 海豚、鲸鱼以及水生哺乳动物睡眠都极少，这一事实可以给睡眠的根本功能提供有用的线索。 海洋中是没有藏身之处的。 会不会是这样，睡眠不但不增加动物受伤害的可能性，反而是减少了这种可能性呢?佛罗里达大学的wils wbb和伦敦大学的ray mddis认为情况就是如此。 可以想像得出，在危险的时刻，那些由于太愚笨而不能自动保持安静的动物，会不由自主地变得动弹不得。 这一点在食肉动物的幼兽身上表现得特别明显。 这是一个很有意思的看法，它至少部分是正确的。