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 with the hours of darkness. Our present concern is with how easily and to what extent this【C1】________can be modified.

The question is no mere academic one. The ease with【C2】_____people can change from working in the day to working at night is a【C3】__of growing importance in industry where automation【C4】__round-the-clock working of machines. It normally takes from five days to one week for a person to adapt to a【C5】__routine of sleep and wakefulness, sleeping during the day and working at night.【C6】__it is often the case in industry that shifts are changed every week. This【C7】__that no sooner has he got used to one routine than he has to change to another, so that much of his time is spent neither working nor sleeping very【C8】_____.

Recent research has shown that people on such systems will revert to go back to their【C9】_____habits of sleep and wakefulness during the weekend and that this is quite enough to destroy any【C10】_____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 habit may persist through all weekends and holidays.

【C6】