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Work hours affect sleep because the sleep behavior of people on nonworkdays differs from that on workdays. For most workers, workday sleep is shorter than nonworkday sleep. On nonworkdays most workers sleep at night. Work schedule interacts with the effects of work hours on workday sleep. On workdays, people on the nightshift sleep the least, people on the afternoon/evening shift sleep the most, and people on the dayshift sleep less than those on the afternoon/evening shift but more than those on the nightshift. Thus, the infradian sleep strategy elected by a shiftworker is ordinarily related to the schedule worked. Despite historical preconceptions that nightshift work disturbs sleep, the primary problem appears to be a workday reduction in sleep length, which most workers do not usually compensate for by getting more sleep on their nonworkdays. Polysomnographic studies of workers on the nightshift show ultradian sleep-stage sequencing similar to that of people who have chronically reduced their usual sleep duration. This chronic reduction in sleep length is evident in data from experienced night shiftworkers, including those who most prefer to work nights. Experienced night shiftworkers also manifest decrements in performance of the sort expected of people suffering from chronic sleep deprivation. It seems reasonable to suggest that chronic sleep deprivation of night shiftworkers may often be a productivity, safety, or health hazard. Sleep disorders may be a secondary problem also facing night shiftworkers. In general, shiftworkers who do night shiftwork report difficulty falling asleep or staying asleep at rates higher than workers who do not work nights. Data suggest that this may be due to the regular practice of inappropriate infradian sleep strategies or to chronic napping habits. At this point in our study of shiftworker sleep behavior, there is as yet no clear evidence that night shiftworkers suffer from clinical sleep disorders at a higher rate than other workers. It is reasonable to suggest that occupational health professionals can make initial assessments of the sleep complaints made by night shiftworkers. Subjective measurements can be used for these assessments if care is taken to use appropriate methods. It is recommended that this data be collected by asking workers quite literal and concrete questions about the time of day they usually elect to go to sleep and get up. A sleep survey of this sort should gather main sleep period and napping times for both workdays and nonworkdays, so that the infradian sleep strategy of the worker can be identified.(ABSTRACT TRUNCATED AT 400 WORDS)