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．

A. change
B. return
C. adaptation
D. tendency