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

睡覺是個特別自然的事兒，你大概從來沒有問過自己為什么會睡覺。

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.

但是從科學的角度來看，這是很神秘的一件事。而事實上，對于睡眠研究人員來說，這仍是該領域中一個懸而未決的問題。

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

停下來仔細想想。我們為什么會睡覺？是因為我們累了嗎？

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?

但是即使我們休息夠了，晚上還是要睡覺。此外，如果讓你設計一種動物，你會讓它每24小時陷入無意識狀態一段時間嗎？

Shouldn't that put it at a terrible disadvantage?

那不是讓它處于很大的劣勢嗎？

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.

毫無疑問針對于這個問題會有很多不同的答案，但我們單去考慮一個能夠實現的整體目標：睡覺是一種生存策略。

Human beings are "diurnal" creatures.

人類都是白天活動的。

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

那意味著我們通常是白天活動力比較強，那個時候我們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，人類的活動力相對差些。我們看不太清物體，我們的顏色辨別也完全消失了，而且我們還沒有其他動物那么敏銳的嗅覺和聽覺。

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.

所以在那些夜間活動的動物們出沒覓食的危險時刻，我們保持原地不動，確實是很有道理的。

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.

一個確保我們不會跌跌撞撞，不會迷路或者被吃掉的穩妥方式就是，每天休息七到八個小時，天亮了我們再起床活動，這樣我們的生存機會會更好地得到保障。

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.

這還不是我們睡覺的唯一原因；但就人類進化而言，它可能是其中一個首要原因——甚至還可能是最重要的一個原因。