Department of Physiology
Hokkaido University School of Medicine
Sapporo 060, Japan.
here seems to be a general agreement that the circadian rhythms in rectal temperature and plasma melatonin in humans are regulated by a self-sustained oscillator similar to the circadian pacemaker located in the suprachiasmatic nucleus in rodents. On the other hand, the oscillation mechanism for the human sleep-wakefulness rhythm is a matter of debate. Two models have been proposed, a mutually coupled self-sustained dual oscillator model and a homeostatic two process model. One of the most striking difference in these two models is that the former assumes mutual interaction between sleep and other circadian rhythms, whereas the latter does not consider the feedback from sleep or wakefulness onto the circadian oscillation. The present study was done to know whether or not a forced sleep schedule affects the plasma melatonin rhythm in attempting to examine the effect of the sleep-wakefulness rhythm on the circadian pacemaker. Previous studies did not separate the effect of sleep or wakefulness from that of light which was associated inevitably with a sleep-wake alternation.
The experiments were carried out in a temporal isolation facility under dim light conditions. The subjects (20-26 yr old) spent two to three weeks in the facility without knowing the time of day. The light intensity in the facility was set below 200 lx. In some experiments, the intensity was reduced to 5 lx. Plasma melatonin rhythm was determined occasionally, and rectal temperature rhythm was continuously. Sleep was monitored polysomnographically. The times going to bed and wake-up were announced by phone. The time allowed for the subjects to sleep was 8 hours. During the waking period, subjects were required to do several tasks.
When the forced sleep schedule was given during free-running, the circadian melatonin rhythm was changed in the half of subjects examined, and entrained by it in two. Although the light intensity was reduced to 5 lx, the melatonin rhythm in several subjects was reset by the forced sleep schedule. On the other hand, the sleep-wakefulness rhythm was entrained by the forced sleep schedule independently from the melatonin rhythm, and started to free-run from the phase determined by the schedule. It took several days to re-establish an internal synchrony between sleep and melatonin rhythm.
It is concluded that a forced sleep schedule is a strong zeitgeber for the human sleep-wakefulness rhythm and for the melatonin rhythm to a lesser extent. There seems to be a mutual interaction between the sleep-wakefulness rhythm and the melatonin rhythm.
