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 (1)_______the hours of darkness. Our present concern is with how easily and to what extent this (2)_______can be modified. The question is no mere academic one. The ease with which people can change from working in the day to working at night is a (3)______of growing importance in industry where automation (4)_______round-the-clock working of machines. It normally (5)______from five days to one week for a person to adapt to a (6)_______routine of sleep and wakefulness, sleeping during the day and working at night. (7)_______, 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 (8)_______ he has to change to another, (9)_______much of his time is spent neither working nor sleeping very (10)_______. One answer would seem to be (11)_______periods
A. wakefulness
B. sleep
C. preference
D. habit
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