Dual oscillator structure of human biological clock: Oscillation coupling between the circadian rhythms and sleep-wake cycle and the sites of oscillation

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1. Non-photic entrainment of sleep-wake rhythm1) Transient periods for entrainment of sleep-wake rhythm to the non-photic zeitgeberThe human subjects were foreced to the sleep-wake schedule for 8 days with was followed by a free-run session of 4 days. In most subjects, plasma melatonin rhythms demonstrated phase-delay shifts during the forced schedule, indicating that the melatonin rhythm failed to entrain to the non-photic zeitgeber. On the other hand, in all subjects examined (n=8) the sleep-wake rhythm started to free-rum from the phase where sleep was allowed by the forced schedule, indicating that the sleep-wake rhythm had been etrained by the non-photic schedule.In order to know the cycle needed for the sleep-wake rhythm to entrain to the non-photic zeitgeber, the exposere period to the forced schedule was shortened to 4 days and further 1 day. As a result, the sleep-wake cycle of a half of the subjects entrained to the forced schedule of 4 days, while that of the other half did … More not. To the forced schedule of 1 day, the sleep-wake rhythm in all subjects failed to entrain. The findings indicate that there was a transient period of several days for the entrainment, and that the entrainment was an oscillatory nature.2) Reestablishment of internal synchronization between the melatonin rhythm and sleep-wake rhythmAfter the forced scheduled was termimated, the internally decynchronized sleep-wake and melatonin rhythms were resynchronized in a few days by means of phase-advance or -delay shifts of sleep-wake rhythm. The amount as well as the direction of phase shift depended on the phase-relashionship before the phase-shift between the sleep-onset and the peak of plasma melatonin rhythm. Namely, when the phase difference of the two rhythms was 2 to 4 hours, there was no phase-shift occurred . On the other hand, the phase difference was 8 to 12 hours, the procuded phase-shift exceeded 6 hours. These findings strongly suggested that there was an oscillatory interaction between the sleep-wake and plasma melatonin rhythms.2. Light responsiveness of human SCNLight responsiveness of the human SCN was studied by means of functional MRI. Eight subjects were exposed to light of 1,000 lx and the changes of neuronal activites in the SCN region were monitored. Only one responded to light stimuli but others not, suggesting that that the SCN response to light is not very strong in humans. Less