Thermosensitivity of the circadian timing system

It is well known that the circadian timing system is very sensitive to environmental light, which is not surprising given the continuous cycling of environmental light during the evolution of species and their biological clocks. Much less attention has been paid to the effect of temperature cycles on the circadian clock; these have, of course, also been present, coupled to light cycles, throughout the evolution of life. The present review addresses thermosensitivity of the neuronal substrate of the biological clock, and of the oscillations in behavior and physiology driven by the clock in ectotherms and homeotherms, which include humans. This overview of findings indicates that the biological clock is sensitive not only to local brain temperature but also, and possibly differentially, to skin temperature. Future research should be aimed at discerning the effects of these two thermal inputs on clock mechanisms. Thermosensitivity may be less pronounced than photic sensitivity. However, with a better understanding of the details of the circadian timing system thermosensitivity, it may prove feasible to apply temperature-cycle entrainment in order to ameliorate sleep-wake rhythm disturbances in blind, elderly and demented subjects that lack appropriate entrainment to light cycles. © 2003, Japanese Society of Sleep Research.