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; some may have hallucinations . There are no rules about sleep. Generally speaking, grown-ups sleep about 7 and a half hours each night and probably more than 60 per cent get between seven and eight hours. But perhaps eight per cent are quite happy with 5 hours or less, and four per cent 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 teenage. Sleep patterns also tend  to be different in the elderly, 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. According to the passage, some people are unable to think clearly because   _   .
they have hallucinations they feel sleepy during the working hours they don't have enough sleep they are certain to be kept from going to bed