Night shift workers suffer from poor sleep, fatigue, gastrointestinal disturbances, and impaired performance. Other consequences are diminished job and public safety. Most of these problems are due to the face that the endogenous circadian clock of the worker does not phase shift to adjust to the night work and day sleep schedule. The 24-hr time cues of the world, such as the natural light-dark cycle, keep the clock from phase shifting. We plan to continue basis research on the use of high intensity "bright" light to phase shift the circadian clock. The protocols are set up to mimic night shift work, so that the results will be more directly applicable to shift work. Subjects stay awake during several 8-hr night shifts, and are exposed to various patterns of bright light during the night shifts. They live at home and go to sleep during the daytime after the night shifts. Thus, they are exposed to the same conflicting time cues as read shift workers. In this sense these are field studies as opposed to laboratory studies in which subjects are shielded from the natural light-dark cycle. In all studies, the circadian rhythm of body temperature is used as a marker for the phase of the circadian clock, photosensors measure light exposure, and questionnaires provide estimates of sleep duration, fatigue and mood. We propose five studies. 1) A study comparing different patterns of bright light (moving and stationary) to determine which are better for phase shifting circadian rhythms. 2) A study of whether medium intensity light can be used to phase shift circadian rhythms. 3) A study of whether bright light occurring at the wrong time during the night shift can keep the rhythms from phase shifting. 4) A study of whether adjustment to the night shift produced by bright light will interfere with subsequent adaptation back to the day shift. 5) A study of whether exercise changes the phase shifting effects of bright light and whether exercise alone can be used to phase shift rhythms.