第6部分：完形填空（第51～65题，每题1分，共15分）
下面的短文有15处空白，请根据短文内容为每处空白确定1个最佳选项。
Sleep
The normal human daily cycle of activity is of some 7-8 hours’sleep alternation with some 16-17 hours’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.
51.A.in B.with C.of D.over
52.A.cycle B.period C.circle D.round
53.A.problem B.difficulty C.trouble D.matter
54.A.asks B.invites C.calls for D.reacts to
55.A.takes B.spends C.demands D.asks
56.A.former B.returned C.reversed D.regular
57.A.Therefore B.Unfortunately C.In a word D.In comparison
58.A.as B.when C.then D.than
59.A.though B.so that C.while D.as
60.A.efficiently B.good C.easily D.happily
61.A.shorter B.better C.longer D.nicer
62.A.So B.In short C.Similarly D.However
63.A.new B.normal C.temporary D.favorite
64.A.change B.return C.adaptation D.tendency
65.A.wakefulness B.sleep C.preference D.habit