阅读理解。

　　Experts say that students usually need eight to ten hours' sleep at night, but more Chinese students do not get enough sleep．Some Chinese parents are usually glad to see their children studying late．They will think their children work very hard．But not all parents are happy about this．Once a mother told us that every morning her 10-year-old boy put up one finger with his eyes still closed, begging for one more minute to sleep．Like thousands of student“early birds”in China, he has to get up before six every morning．A report shows that if they don't have a good night's sleep, students seem to be weaker than they should be．Many students have fallen asleep during class at one time or another．Too much homework is not the only reason(原因) why students stay up late．Some watch TV or play the computer games late into the night．

　　Experts have ever said that students should develop good study habits(习惯)．So some clever students never study late, they are able to work well in class．

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　　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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