Integrated model incorporating circadian phase dynamics and the thermoregulatory mechanism of sleep

Recently, physiological findings have suggested the existence of an integrated regulatory mechanism for sleep-wake rhythms, as follows. Homeostatic regulation of the sleep-wake pattern and body temperature involves circadian oscillators and the history of activity patterns. In addition, the rest-activity pattern can modulate the dynamics of circadian oscillators via feedback mechanisms. We have developed models of sleep-wake rhythms based on the thermoregulatory function of sleep and the multi-oscillator mechanism with feedback. The former model is tentatively called the "thermoregulatory model"and the latter the "multi-oscillator model". The application of the thermoregulatory model is confined to internally or externally entrained situations in which steady circadian oscillation and fixed-phase relationships between the oscillators can be expected to be retained. On the other hand, prediction by the multi-oscillator model is confined to the dynamics of oscillator phase, since the model consists of phase oscillators without amplitude. Making use of the mutual complementarity of these two models, an integrated model of the regulatory mechanism of sleep-wake rhythms was obtained by combining them. Although the integrated model could be valid under conditions of concurrency in which the sleep-wake/rest-activity patterns of the two models can be considered rhythmic and synchronized, the results of a simulation show that the model can be applied to predict the behavior of sleepiness as well as circadian oscillators in a wide variety of situations, such as internally or externally entrained steady states, trans-meridian flight and rotating shift work. The integrated model could provide a general framework for exploring possible physiological and dynamical regulatory mechanisms of sleep-wake rhythms as well as appropriate design of work schedules.