[Non-photic entrainment of human circadian clock--effects of forced sleep-wake schedule on the circadian rhythm in plasma melatonin].

The present study was performed in order to have better understanding of the human circadian system. Two hypotheses concerning with the human circadian system have been advanced. One is a two self-sustained oscillator hypothesis, and the other is a two process hypothesis. Both hypotheses are based on a phenomenon called internal desynchronization. A major difference between the two hypothesis is that sleep or wakefulness does not affect the circadian oscillation in the two process hypothesis, while a mutual interaction between sleep (wakefulness) and the circadian oscillation is not excluded in the two oscillator hypothesis. On the other hand, social sues such as a sleep-wake schedule and regular contact with others have been considered as a major time cues (zeitgeber) for the human circadian rhythm. However, lights brighter than 5,000 lux was recently demonstrated to be capable of resetting the human circadian rhythm, and it is now a matter of debate whether the resetting effect of social cues is due to the light-dark cycle inevitably associated with a sleep-wake schedule. In the present study, the effects of forced sleep-wake schedules on the circadian rhythm in plasma melatonin and rectal temperature were examined in subjects under temporal isolation, which are not explained by the two process hypothesis. As a result, the forced sleep-wake schedule of 24.0-hour period in dim light condition (below 200 lux) entrained the circadian rhythms in 3 out of 8 subjects. The forced schedule in extremely dim light condition (below 5 lux) was also capable of entraining the circadian rhythm. On the other hand, the forced schedule of 23.5-hour period showed no entrainability. It is concluded that the non-photic entrainment is operating in the human circadian clock, which supports the two self-sustained oscillator hypothesis.