We all know that the normal human daily cycle of some 7-8 hours sleep alternating(交替) with some 16-17 hours wakefulness and that, broadly speaking, the sleep normally happens in the same period with the hours of dark- ness. Our present concern is 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 for industry which calls for round-the-clock working of machines. It normally takes from five days to one week for a person to adapt to a reverse of sleep and wakefulness, sleeping during the day and working at night. Unfortunately it is often the case in industry that shifts 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 than he has to change to another, so that much of his time is spent neither working nor sleeping very efficiently. One answer would seem to be longer periods on each shift, a month, or even three months. Recent research by Bonjer of the Netherlands, however, shows that people on such systems will return to their normal habits of sleep and wakefulness during the weekend and that this is quite enough to destroy any adaptation to night work built up during the week. The only real solution appears to be to hand over the night shift to a corps of permanent night workers whose night wakefulness may persist through all weekend and holidays. An interesting study of the domestic life and health of night-shift workers was carried out by Brown. She found a high incidence of disturbed sleep, digestive disorder and domestic breakup among those on alternating day and night shifts, but no abnormal occurrence of these symptoms among those on permanent night work. The question raised in Para. 1 is no mere academic one because______.