Humans, like most of the plants and animals on the planet, have an internal circadian clock which produces daily (circadian) rhythms in almost all functions and variables such as body temperature, cognitive and physical performance, the secretion of hormones and neurotransmitters, etc. Our circadian clocks are synchronized to the 24 hour day primarily by the light and dark to which we are exposed, to the 24 hour light-dark cycle. As diurnal animals we are programmed by our circadian clocks to feel sleepy and to sleep at night, and to be awake and alert during the day. Body temperature, alertness and performance reach a low point around 4 to 6 am. This creates a problem for night shift workers who feel sleepy and are most prone to accidents at work, especially near the end of the night shift. This low point produced by the circadian clock persists even if the worker obtains enough sleep during the day. Furthermore, since we are programmed to be alert during the day, night shift workers often have shortened and disrupted sleep when they go home to sleep during the daytime after the night shift. This results in cumulative partial sleep deprivation which exacerbates the natural circadian dip in alertness during the night shift. Aside from the obvious safety risks to the night worker and to the public, especially for critical jobs such as nuclear power plant operators and hospital intensive care nurses, there are long term health consequences for the workers. Night shift workers have a higher incidence of cancer, diabetes and heart disease. There are several factors that may account for this. One is the fact the night workers are forced to be awake, to eat meals and to sleep at the wrong times according to their internal circadian clock. Thus, the body is not properly prepared for these activities. This is called "circadian misalignment" because the daily activities and the internal circadian rhythms are not properly aligned. Another factor which could account for health problems is the shortened and disrupted sleep. Shorter sleep has been linked to obesity and diabetes. Finally, the higher incidence of cancer could be caused, in part, by light during the night shift. Light suppresses the secretion of the neurohormone melatonin which is an antioxidant. In any case, the International Agency for Research on Cancer has classified night work as a probable carcinogen. Millions of Americans, 15 to 20% of the workforce, work night shifts. This number will probably increase as we become even more of a 2417 society. For over 20 years the P.I. has been working on methods to trick the circadian clock of the night worker into aligning with the night work, day sleep schedule in order to eliminate circadian misalignment. She has shown that by creating a new 24 pattern of light and dark the circadian clock can be completely reset, or phase-shifted, to align with the night work, day sleep schedule. This is done by using bright light from light boxes during the night shift, dark sunglasses on the commute home and a regular, dark, daytime sleep period. Most of these studies, including those reported here, were simulated night shift studies in which volunteers "work" night shifts in the lab. But they sleep at home like real night shift workers. The circadian clocks of most real night workers do not phase shift to adjust to the night work schedule, because they are not exposed to an appropriate pattern of light and dark. For example, they do not have bright light at work, do not wear dark sunglasses on the way home, and do not maintain a regular dark sleep episode after work. One problem with introducing these methods into the workplace is that complete alignment to the night shift schedule leaves the individual misaligned with their friends and family on days off. we expect that night workers might not adhere to a program that makes them feel good during work days, but not on days off. The 5 year project reported here was designed to assess the feasibility of achieving and maintaining a compromise circadian phase position for permanent night shift work. (This procedure could easily be expanded for slowly rotating shift work systems.) A compromise phase position is one in which the circadian clocks of night workers are partially aligned with a night work and day sleep schedule, so that their sleepiest time is delayed out of the night work period and into the first part of the daytime sleep episode. Because the sleepiest time is shifted out of the night work period, alertness and performance during the work period will improve. Because the sleepiest time of day is shifted into the daytime sleep period after work, sleep will improve. A late sleep schedule is maintained on days off, which is about half-way between the earlier sleep schedule of the nonshift working society and the very late (daytime) sleep schedule the night worker is forced to adopt after night-shifts. With this days-off sleep schedule, which is another compromise, the sleepiest time of day will occur near the end of the sleep period, producing fairly good sleep on days off. An added bonus is that the time for the secretion of melatonin is shifted from primarily during the night shift to primarily during the dark sleep periods, so that melatonin synthesis is not suppressed, thus reducing the cancer risk. Our overall goal was to produce a circadian phase position that is good for working at night and for both daytime sleep after night shifts as well as late nighttime sleep on days off. These studies assessed the circadian phase of experimental groups and control groups during a series of blocks of night shifts (3-5 consecutive night shifts) alternating with 2 consecutive days off. Experimental subjects were exposed to intermittent bright light pulses during night shifts (usually only 15 minutes of bright light per hour, only 4 to 5 pulses per night shift). The light pulses were produced by light boxes usually sold for SAD or winter depression. The subjects had scheduled sleep episodes at home after night shifts and on days off. Bedrooms were made dark by covering the windows with black plastic. Subjects wore dark sunglasses during their morning commute home from night shifts. The dark sunglasses are commercially available, meeting the traffic signal color requirements of ANSI 280.3-1996, and are general purpose sunglasses for driving and most outdoor activities. Subjects were required to go outside for afternoon light exposure following their daytime sleep after night work and their sleep on days off. This afternoon light exposure (a "light brake") was intended to keep their circadian clocks from completely adjusting to the night work and day sleep schedule, since partial adjustment was the goal of the compromise phase position. Like real night shift workers, control subjects remained in normal room light during their night shifts, and had unrestricted sleep and outdoor light exposure. They were given lightly tinted sunglasses. These interventions reset the circadian clock of most of the experimental subjects to the target compromise phase position after about a week. This first week consisted of 3 night shifts, 2 days off and then more night shifts. when the circadian clocks of the subjects had shifted and were close to the compromise phase position, the subjects had improved alertness, performance, and mood during night shifts. In many subjects performance levels returned to daytime baseline levels, which is a vast improvement that cannot be matched even by the use of stimulants during the night shift. Experimental subjects were able to sleep for nearly all of the allotted time in bed on the prescribed schedule. In other words, they slept as long as permitted after the night shift (which was 7 hours) and as long as permitted on days off (which was 9 hours). In contrast to the experimental subjects, the sleepiest circadian time for most control subjects remained during night work, and was accompanied by impaired performance and mood, as well as decreased daytime sleep duration. A few control subjects adopted sleep schedules similar to those required of the experimental subjects, or even more extreme schedules with very late sleep on days off. Some of these subjects reset their circadian clocks as much as the experimental subjects even without bright light during the night shifts or very dark sunglasses during the commute home. However, the most reliable way to reduce circadian misalignment is to use bright light at work and wear dark sunglasses on the way home, along with the recommended sleep schedule. These findings indicate that complete circadian adjustment to a night work and day sleep schedule is not necessary to produce substantial improvements in alertness and performance during night work. Instead, practical interventions that could be adopted by night workers with the cooperation of their employers can produce partial adaptation to a compromise circadian phase position. This will improve night shift alertness and performance while permitting sufficient sleep after work and on days off Future studies should implement these techniques into the workplace. This will take the cooperation of employers who would have to provide light boxes or overhead fixtures, but in return they could see increased productivity and reduced mistakes and accidents. It will also require some life style changes of the workers, including giving up morning activities after the night shift and on days off Both employers and workers would have to agree to eliminate rapidly rotating shift work schedules, because such schedules do not permit circadian alignment. Finally, it will require a change in the culture to recognize the importance of obtaining enough sleep and getting it at the right time. Night shift workers need the cooperation of family and friends to be able to adopt the recommended sleep schedules. Fortunately, there are organizations such as the National Sleep Foundation, which have begun to educate the public about the importance of sleep. Their public awareness campaigns may be responsible for the increasing number of news stories about sleep. Adequate sleep has finally been recognized as important in the list of lifestyle changes that can improve the health of our nation, along with smoking cessation, diet and exercise.