No one knows why we sleep, but it's certain that we need to.
没有人知道为什么我们要睡觉，但是睡觉是我们必然需要做的事情。
(Read about secrets of why we sleep in National Geographic magazine.)
(请参考国家地理杂志《睡眠的奥秘》一文)。
This explains why we sleep at all and this probably explains why we sleep at night.
这个解释了我们为什么要睡觉，这个解释了我们为什么在晚上睡觉。
Over the years, scientists have come up with a lot of ideas about why we sleep.
关于我们为什么要睡觉，多年来，科学家提出了很多想法。
This seemed a fair explanation of why we sleep at night, and why we snooze on rainy Sunday afternoons.
这看起来好像是一个为什么我们夜间睡觉，或下雨的星期日下午打盹的合理解释。
我们为什么要研究睡眠？
One night, my sister wanted to sleep in the room to see why he woke up screaming every time we put him in his crib fast asleep.
一天，我姐姐打算睡在那个房间，想知道为什么每次我们把他放在婴儿床上熟睡时，他就会尖叫着醒来。
我们为什么需要最佳的睡眠？
Why does the rustle of sheets wake us up on some nights, but we sleep through the sound of our alarm clocks going off on others?
为什么我们在某些夜晚会被床单发出的沙沙声吵醒，却没有理会那些吵醒别人的闹钟声而睡过头呢？
We didn't ask 'Why didn't you get enough rest or sleep?' said Lela McKnight-Eily, an epidemiologist for the Centers for Disease Control and Prevention who led the study.
负责此项研究的美国疾病防控中心的流行病学家莱拉•麦克耐特-艾莉说：“我们并没有提出‘为什么你没有得到足够的休息或者睡眠？’ 这一问题。”
There are a few theories as to why we need so much sleep.
关于为什么我们需要这么多睡眠有一些理论。
The findings may explain why parents instinctively rock babies to sleep, and why we find it so easy to nod off when slumped in a rocking chair, they add.
他们还说，这项发现或许可以解释父母本能的摇晃婴儿睡觉的原因，也可以解释为何我们一躺在摇椅上就会打盹。
Why do we sleep and why don't we — why aren't we always awake?
我们为什么会睡觉，我们为什么不是一直醒着？
At first we couldn't figure out why they made so much noise when it was time for them to go to sleep, but it was because someone had accidentally taken the toy from their basket.
起先我们不理解为什么睡觉时它们喜欢制造一些噪音出来，后来才明白原来是有人从它们的篮子里拿走了这个玩具。
That could explain why our sleep-deprived selves are so cognitively challenged: We are, if not precisely half-asleep, partially asleep.
这解释了为什么我们缺少睡眠时，认知能力会下降：这时的我们即使不是半睡状态，也是部分睡眠。
So why do we dream? To protect our sleep.
所以，“我们为什么要做梦”的答案就是：为了保证我们的睡眠。
Sounds interesting, but why exactly do we want to put our threads to sleep?
听上去很有趣，但究竟为什么要让线程睡眠呢？
That's probably why we have so many expressions to describe sleep.
这可能就是我们为什么有那么多好词语来描绘睡觉。
Sleep and dreams have interested people for centuries. Scientists are learning more about sleep, but they still know very little about why we dream.
几个世纪以来，睡觉和做梦都让人着迷。科学家对于睡觉了解了很多，但是他们仍然不清楚为什么人会做梦。
We don't know why we need sleep but we regain our vigor after sleep.
我们不知道自己为什么要睡觉，可是我们睡了之后精神就恢复了。
为什么我们需要睡眠？
The truth is, even scientists can't agree on why we need to sleep.
事实是甚至科学家们对我们为什么要睡觉也不能达成共识。
Why do we not doubt whether our thoughts and actions are not another sort of dreaming, and our waking a certain kind of sleep?
为什么我们不怀疑我们的想法和行动只不过是另一种梦？以及，我们的清醒只是某种形式的睡觉？
Why do we close our eyes when we sleep, when we cry, when we imagine, when we kiss? Because the most beautiful thing in the world is unseen.
为什么我们闭上眼睛，当我们睡觉，当我们哭泣，当我们想像，当我们亲吻？因为，世界上最美丽的东西是看不见的。
One theory for why we need sleep is that our brains use the opportunity to consolidate memories from that day.
关于为什么人类需要睡眠的一个理论是，我们的大脑利用这个机会整合一天的记忆。
One theory for why we need sleep is that our brains use the opportunity to consolidate memories from that day.
关于为什么人类需要睡眠的一个理论是，我们的大脑利用这个机会整合一天的记忆。
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