How many phases of sleep do people usually cycle through?

There must be few questions on which responsible opinion is so utterly divided as on that of how much sleep we ought to have. There are some who think we can leave the body to regulate these matters for itself. "The answer is easy," says Dr. A. Burton. "With the tight amount of sleep you should wake up fresh and alert five minutes before the alarm tings." If he is right many people must be under sleeping, including myself. But we must remember that some people have a greater inertia than others. This is not meant rudely. They switch on Slowly, and they are reluctant to switch off. They are alert at bedtime and sleepy when it is time to get up, and this may have nothing to do with how fatigued their bodies are, or how much sleep they must take to lose their fatigue.

Other people feel sure that the present trend is towards too little sleep. To quote one medical opinion, "Thousands of people drift through life suffering from the effects of too little sleep; the reason is not that they can't sleep. Like advancing colonists, we do seem to be grasping ever more of the land of sleep for our waking needs, pushing the boundary back and reaching, apparently, for a point in our evolution where we will sleep no more. This in itself, of course, need not be a bad thing. What could be disastrous, however, is that we should press too quickly towards this goal, sacrificing sleep only to gain more time in which to jeopardize our civilization by actions and decisions made weak by fatigue. Then, to complete the picture, there are those who believe that most people are persuaded to sleep too much. Dr. H. Roberts, writing in Every Man in Health, asserts it may safely be stated also. It would be a pity to retard our development by holding back those people who are gifted enough to work and play well with less than the average amount of sleep, if indeed it does them no harm. If one of the trends of evolution is that more of the life span is to be spent in gainful waking activity, then surely these people are in the van of this advance. The author seems to indicate that ______. A．there are many controversial issues like the right amount of sleep B．among many issues the right amount of sleep is the least controversial C．people are now moving towards solving many controversial issues D．the right amount of sleep is a topic of much controversy among doctors

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The normal human daily cycle of activity is of some 7~8 hours' sleep【1】with some 16 —17 hours' wakefulness and that the sleep normally coincides【2】the hours of darkness. Our present【3】is with how easily and to what extent this【4】can be modified.

The question is no mere academic one. The ease with【5】people can change from working in the day to working at night is a【6】of growing importance in industry where automation【7】round-the-clock working of machines. It normally【8】from five days to one week for a person to adapt to a【9】routine of sleep and wakefulness, sleeping【10】the day and working at night.【11】it is often the case in industry that shifts are changed every week. This【12】that no sooner has he got used to one routine【13】he has to change to another,【14】much of his time is spent neither working nor sleeping very【15】 One answer would seem to be【16】periods on each shift, a month, or even three months.【17】, recent research has shown that people on such systems will revert to go back to their【18】habits of sleep and wakefulness during the week-end and that this is quite enough to destroy any 【19】to night work built up during the week. The only real solution appears to be to hand over the night shift to those permanent night workers whose【20】may persist through all week-ends and holidays. (1) A．allegation B．alternation C．allocation D．alternative

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How many phases of sleep do people usually cycle through?

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