More than one-third of working men and one-quarter of working women in the U. S. report being exposed to a variable schedule that includes both day and night work. Of these, approximately 7.3 million must regularly work overnight, either on permanent night shifts or rotations between day, evening, and night shifts, requiring them to forego nocturnal sleep and attempt to sleep during the day. Despite this nocturnal deprivation of sleep, these workers typically experience daytime insomnia, leading to diminished alertness and cognitive performance and increased sleep tendency during waking hours at night. In fact, 55 percent of night shift workers report nodding off or falling asleep at work at least once per week, with more than 30 percent reporting that such incidents occur more than three times per week. Recent research has demonstrated that properly timed exposure to bright light and darkness can rapidly reset the human circadian pacemaker that-controls the timing of the sleep-wake cycle, enabling the circadian pacemakers of individuals working at night to fully adapt to their desired schedules within 2-3 days. On the basis of these results, four testable hypotheses are proposed: (l) that bright light can rapidly shift the endogenous circadian rhythms of plasma melatonin secretion of individuals working at night, such that their circadian timing system remains adapted to their inverted sleep-wake schedule; (2) that bright light can increase sleep efficiency, and reduce the number and duration of awakenings during the daytime sleep of individuals working at night; (3) that bright light can reduce the frequency of involuntary microsleep episodes and decrease sleep propensity during scheduled wakefulness at night; and (4) that bright light can improve the alertness and cognitive performance of individuals working at night. An experiment is proposed to evaluate the impact of bright light treatment on the daytime sleep and nocturnal cognitive performance of individuals during a combined laboratory- and field-based simulation of a complete, 3- month shift rotation. Subjects exposed to an appropriate schedule of bright light while working in the laboratory will be compared to a control group of subjects without bright light exposure. Sleep, activity, and light exposure will be monitored with ambulatory recording devices, circadian phase will be assessed from serum melatonin levels, and cognitive performance will be evaluated with a computer-administered battery of tests. This work has significant implications for shiftworker health and population safety. Shiftwork is associated with cardiovascular disease, depression, increased drug use, and digestive disorders. Many catastrophic accidents (e.g., Exxon Valdez, Bhopal, Three-Mile Island, Chernobyl) have occurred during the night shift with fatigue identified as a contributor. An effective countermeasure to the personal and societal risks of shiftwork could substantially improve public health and safety.