extend back as far as the reptiles.
There is some evidence that the two types of sleep, dreaming and dreamless,
depend on the life-style of the animal, and that predators are
statistically 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 immobilized 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 dolphins 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
vulnerability, the function of sleep is to decrease it? Wilse Webb of the
University of Florida and Ray Meddis of London University have suggested
this to be the case. It is conceivable that animals who 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 predatory animals. This is an interesting notion and
probably at least partly true.
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
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 Wilse
Webb 和伦敦大学的 Ray Meddis 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。
