This simulated night shift field study compared high-intensity ("bright") light exposures designed to either facilitate or conflict with adaptation to a 9-h phase shift of the sleep/dark schedule. There were 7 days of baseline with night sleep followed by 8 night shifts with day sleep in a 2 x 2 design with factors bright light (facilitating vs. conflicting) and direction of shifted sleep/dark (delayed vs. advanced). A total of 32 subjects (8 in each group) were exposed to 3 h of bright light (about 5,000 lux) and 5 h of ordinary indoor room light of "dim" light (< 500 lux) during each 8-h night shift. The bright light was timed according to the light phase-response curve (PRC) to delay or advance rhythms; it was timed to occur either before or after the baseline body temperature minimum, which served as an estimate of the PRC crossover point between delays and advances. Core body temperature was measured continuously and demasked to determine daily temperature minima. Significantly more subjects showed large temperature rhythm phase shifts (> or = 6 h during the last 4 night shifts relative to baseline) with facilitating bright light compared to conflicting bright light as well as with delayed sleep/dark compared to advanced sleep/dark. The combination of facilitating bright light and delayed sleep/dark produced large phase delay shifts in all subjects tested. By contrast, the combination of conflicting bright light and advanced sleep/dark resulted in very small phase shifts in most subjects. Because bright light timed to delay usually was not able to phase shift rhythms when sleep/dark was advanced, it appears that the timing of sleep/dark was as important as the timing of the bright light. There was a relationship between the amount of phase shift and the individual's baseline phase when sleep/dark was delayed. Larger phase delays were achieved by subjects with later baseline temperature minima and greater eveningness on the Morningness-Eveningness Questionnaire. These results show that it is important to time bright light appropriately to achieve circadian adaptation to the night shift and that individual differences play an important role in the ability of the circadian system to phase shift.
