[单项选择]We all know that the normal human daily cycle of activity is 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 with how easily and to what extent this cycle can be modified.
The question is no mere 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 in 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 routine of sleep and wakefulness, sleeping during the day and working at night. Unfortunately, it is often the case in industry that shies 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
A. not to change shifts from one week to the next.
B. to have longer periods on each shift.
C. to employ people who will always work at night.
D. to select people who adapt quickly to work at night.

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