单项选择题

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 on each shift, a month, or even three months. (12)_______, recent research has shown that people on such systems will revert to go back to their (13)_______ habits of sleep and wakefulness during the weekend and that this is quite enough to destroy any (14)_______ 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 (15)_______may persist through all weekends and holidays.