We all know that the normal human daily cycle of activity is of some 7-8 hours' sleep alternating with some 16 to 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 not 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 reversed routine of sleep and wakefulness, sleeping during the day and working at night. Unfortunately, it is often the ease in industry tint 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, has shown that people on such systems will revert 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 nocturnal wakefulness may persist through all weekends 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 discord and domestic disruption among those on alternating day and night shifts, but no abnormal occurrence of these symptoms among those on permanent night work. The article is mainly about ______.