PHASE-SHIFTING HUMAN CIRCADIAN-RHYTHMS - INFLUENCE OF SLEEP TIMING, SOCIAL CONTACT AND LIGHT EXPOSURE

1. Both the timing of behavioural events (activity, sleep and social i nteractions) and the environmental light-dark cycle have been reported to contribute to entrainment of human circadian rhythms to the 24 h d ay. Yet, the relative contribution of those putative behavioural synch ronizers to that of light exposure remains unclear. 2. To investigate this, we inverted the schedule of rest, sedentary activity and social contact of thirty-two young men either with or without exposure to bri ght light. 3. On this inverted schedule, the endogenous component of t he core temperature rhythm of subjects who were exposed to bright ligh t showed a significant phase shift, demonstrating that they were adapt ing to the nerv schedule. In contrast, the core temperature rhythm of subjects who were not exposed to bright light moved on average 0.2 h l ater per day and after 10 days had not significantly adapted to the ne w schedule. 4. The direction of phase shift in the groups exposed to b right light was dependent on the time of bright light exposure, while control subjects drifted to a later hour regardless of the timing of t heir schedule of sleep timing, social contact and meals. 5. These resu lts support the concept that the light-dark cycle is the most importan t synchronizer of the human circadian system. They suggest that invers ion of the sleep-wake, rest-activity and social contact cycles provide s relatively minimal drive for resetting the human circadian pacemaker . 6. These data indicate that interventions designed to phase shift hu man circadian rhythms for adjustment to time zone changes or altered w ork schedules should focus on properly timed light exposure.