It's such a commonplace event that you probably never asked yourself why you sleep.

睡觉是个特别自然的事儿，你大概从来没有问过自己为什么会睡觉。

But from a scientific point of view, this is far from a mundane matter -- in fact, for sleep researchers, it's one of the biggest unanswered questions in the field.

但是从科学的角度来看，这是很神秘的一件事。而事实上，对于睡眠研究人员来说，这仍是该领域中一个悬而未决的问题。

Stop and think about it for a second. Why should we sleep? Is it because we get tired?

停下来仔细想想。我们为什么会睡觉？是因为我们累了吗？

But we sleep every night, even when we have had plenty of rest. Furthermore, if you were designing an animal, would you have it lapse into prolonged periods of unconsciousness every twenty-four hours?

但是即使我们休息够了，晚上还是要睡觉。此外，如果让你设计一种动物，你会让它每24小时陷入无意识状态一段时间吗？

Shouldn't that put it at a terrible disadvantage?

那不是让它处于很大的劣势吗？

There are no doubt several different answers to this question, but let's just consider one overall purpose sleep may fulfill: sleep as a survival strategy.

毫无疑问针对于这个问题会有很多不同的答案，但我们单去考虑一个能够实现的整体目标：睡觉是一种生存策略。

Human beings are "diurnal" creatures.

人类都是白天活动的。

That means we are normally active during daylight hours, when our senses function most effectively.

那意味着我们通常是白天活动力比较强，那个时候我们的感官功能也是最有效率的。

At night, humans do rather poorly: we can't see objects well, our color vision is entirely lost, and we don't have the smelling or hearing acuity of other animals.

晚上，人类的活动力相对差些。我们看不太清物体，我们的颜色辨别也完全消失了，而且我们还没有其他动物那么敏锐的嗅觉和听觉。

So it actually does make sense to have us stay put during the dangerous period when "nocturnal," or night-waking, animals are on the prowl.

所以在那些夜间活动的动物们出没觅食的危险时刻，我们保持原地不动，确实是很有道理的。

And one sure way to make sure we don't stumble around and get lost or eaten is to have us immobilized for seven or eight hours, rising again only when the light is back and our survival chances are better.

一个确保我们不会跌跌撞撞，不会迷路或者被吃掉的稳妥方式就是，每天休息七到八个小时，天亮了我们再起床活动，这样我们的生存机会会更好地得到保障。

It's not the only reason we sleep; but in terms of evolution, it may have been one of the first reasons -- perhaps even the most important one.

这还不是我们睡觉的唯一原因；但就人类进化而言，它可能是其中一个首要原因——甚至还可能是最重要的一个原因。