No one knows why we sleep, but it’s certain that we need to．People who are prevented from sleeping begin to suffer obvious effects after a few days－they think less clearly, and they fall asleep during the working hours．

　　There are no rules about sleep．Generally speaking, grown-ups sleep about 7 and a half hours each night and probably more than 60 percent get between seven and eight hours．But perhaps eight percent are quite happy with 5 hours or less, and four percent or so find that they want ten hours or more．If you feel all right, you’re probably getting enough sleep．The important thing is not to worry how much other people get－their needs may be different．Exercise doesn’t seem to increase the need for sleep－office workers, for example, sleep for about as long as people doing physically active work．

　　Children sleep more than grown-ups－perhaps 14 to 18 hours soon after birth, going down to grown-up levels by early teenagers．Sleep patterns also tend to be different in the old people, who may sleep less at night than they did when younger, find sleep getting more broken, and often make it a rule to sleep during the daytime．

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阅读理解，阅读下面的四篇短文，根据短文内容，从短文后各题所给的A、B、C、D四个选项中，选择最佳选项。

　　Even though global warming may have made the weather more mild, many animals are still hibernating(冬眠)．It's too bad that humans can't hibernate．In fact we almost did．

　　Apparently, at times in the past, farmers in France liked a semi-state of human hibernation．So writes Graham Robb, a British scholar who has studied the sleeping habits of the French farmers．As soon as the weather turned cold, people all over the France shut themselves away and practiced the forgotten art of doing nothing at all for months．

　　In line with this, Jeff Warren, a producer at CBC Radio's The Current, tells us that the way we sleep has changed since the invention of lighting．

　　When historians began studying texts of the Middle Ages, they noticed something referred to as “first sleep”, which was not clarified, though．Now scientists are telling us our ancestors most likely slept in separate periods．The business of eight hours’ continuous sleep is a modern invention．

　　In the past, without the artificial light, humans went to sleep when it became dark and then woke themselves up around midnight．The late night period was known as “The Watch”．It was when people actually kept watch against wild animals, although many of them simply moved around or visited family and neighbors．

　　According to some sleep researchers, a short of insomnia(失眠)at midnight is not a disorder．It is normal．Humans can experience another state of consciousness(清醒状态)around their sleeping, which happens in the brief period before we fall asleep or wake ourselves in the morning．This period can be a creative time for some people．

　　Playing with your sleep rhythms(节奏)can be dangerous, as worry may set in．Medical science doesn't help much in this case．It offers us medicines for a full night's continuous sleep, which sounds natural; however, according to Warren's theory, it is really the opposite of what we need．

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