Sleep is very ancient. We share it with all the primates① and almost all the other mammals and birds： it may extend back as far as the reptiles②.
Dreaming and dreamles ssleep depend on the life-style of the animal， and that predators are statistically③. much more likely to dream than prey， which are in turn much more likely to experience dreamless sleep. In dream sleep， the animal is powerfully immobilized④ and remarkably unresponsive to external stimuli. Dreamless sleep is much shallower， and we have all witnessed cats or dogs cocking their ears to a sound when apparently fast asleep. The fact that deep dream sleep is rare among prey today seems clearly to be a product of natural selection， and it makes sense that today， when sleep is highly evolved， the stupid animals are less frequently immobilized by deep sleep than the smart ones. But why should they sleep deeply at all？ Why should a state of such deep immobilization ever have evolved？
Perhaps one useful hint about the original function of sleep is to be found in the fact that dolphins⑤ and whales and aquatic mammals in general seem to sleep very little. There is， by and large， no place to hide in the ocean. Could it be that， rather than increasing an animal’s vulnerability⑥， the function of sleep is to decrease it？
It is conceivable that animals that are too stupid to be quiet on their own initiative are， during periods of high risk， immobilized by the implacable arm of sleep. The point seems particularly clear for the young of predatory⑦ animals.
① primaten. 灵长类动物
② reptilen. 爬行动物；爬虫类
③ statisticallyadv. 统计上
④ immobilizev. 使不动，使不能移动
⑤ dolphinn. ［动］海豚[C]
⑥ vulnerabilityn. 易受伤，易受责难，弱点
⑦ predatoryadj. 以捕食其他动物为生的，食肉的
睡眠的进化
睡眠是古老的。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。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。会不会是这样的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？
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。