Intermittent bright light and exercise to entrain human circadian rhythms to night work

Bright light can phase shift human circadian rhythms, and recent studies ha ve suggested that exercise can also produce phase shifts in humans. However , few studies have examined the phase-shifting effects of intermittent brig ht light, exercise, or the combination. This simulated night work field stu dy included eight consecutive night shifts followed by daytime sleep/dark p eriods (delayed 9 h from baseline). There were 33 subjects in a 2 x 2 desig n that compared 1) intermittent bright light (6 pulses, 40-min long each, a t 5,000 lx) versus dim light and 2) intermittent exercise (6 bouts, 15-min long each, at 50-60% of maximum heart rate) versus no exercise. Bright ligh t and exercise occurred during the first 6 h of the first three night shift s. The circadian phase marker was the demasked rectal temperature minimum. Intermittent bright-light groups had significantly larger phase delays than dim-light groups, and 94% of subjects who received bright light had phase shifts large enough for the temperature minimum to reach daytime sleep. Exe rcise did not affect phase shifts; neither facilitating nor inhibiting phas e shifts produced by bright light.