题目内容：
请根据短文的内容，回答题。Sleep
The normal human daily cycle of activity is of some 7-8 hours' sleep alternation with some 16-17hours' wakefulness and that the sleep normally coincides __________ (51) the hours of darkness. Our present concern is with how easily and to what extent this __________ (52) can be modified.
The question is no mere academic one. The case with which people can change from working in the day to working at night is a __________ (53) of growing importance in industry where automation __________ (54) round-the-clock working of machines. It normally __________ (55)from five days to one week for a person to adapt to a __________ (56) routine of sleep and wakefulness, sleeping during the day and working at night. __________ (57), it is often the case in industry that shifts are changed every week. This means that no sooner has he got used to one routine __________ (58) he has to change to another, __________ (59) much of his time is spent neither working nor sleeping very __________ (60).
One answer would seem to be __________ (61) periods on each shift, a month, or even three months. __________ (62), recent research has shown that people on such systems will revert to go back to their __________ (63) habits of sleep and wakefulness during the weekend and that this is quite enough to destroy any __________ (64) 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 __________ (65) may persist through all weekends and holidays.
_________ A．in
B．with
C．of
D．over
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