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PASSAGE 16 Sleep We all know that the normal human daily cycle of some 7-8 hours sleep alternating with some 16-17 hours wakefulness and that, broadly speaking, the sleep normally coincides with the hours of darkness. Our present concern is win how easily and to what extent this cycle can be modified. The question is no more academic one. The case, for example, with which people can change from working in the day to working at night is a question of growing importance industry where automation calls insistently for round-the-clock working of machines. It normally takes from five days to one week for a person to adapt to a reversed of sleep and wakefulness, sleeping during the day and working at night. Unfortunately it is often the case in industry that shifts are changed every week; a person may work from 12 midnight to 8 a.m. one week , 8 a.m. to 4 p.m. the next, and 4 p.m. to 12 midnight the third and so on. This means that no sooner has he got used to one routine than he has to change to another, so that much of his time is spent neither working nor sleeping very efficiently. One answer would seem to be longer periods on each shift, a month, or even three months. Recent research by Bonjer of the Netherlands, however, has shows that people on such systems will revert to their normal habits of sleep and wakefulness during the week-end and that this is quite enough to destroy any adaptation to night work built up during the week. | 
