Sleep propensity is modulated by circadian and behavior-induced changes in cutaneous temperature

A close relation between sleep and body temperature has been noted already for a long time. Although a correlation is indisputable, there is at present hardly evidence for a causal involvement of sleep in changes in body temperature. Concerning the reverse, a causal involvement of body temperature in sleep has been demonstrated: if core or skin temperature changes activate thermoregulatory processes aimed at heat loss or heat preservation, sleep is usually disrupted. We have recently proposed that sleep propensity is also affected by more subtle changes in skin temperature, within the thermoneutral range (Van Someren (2000). Chronobiol. Int. 17, 313-354). These changes are likely to modulate the firing properties of thermosensitive neurons in brain areas involved in sleep regulation. Subtle changes in skin temperature occur daily under control of the circadian timing system. They could provide this system with an additional signal pathway to support its neuronal and neurohormonal signals to enforce circadian modulation of sleep propensity. Subtle changes in skin temperature also result from behavior, and could contribute to the changes in sleep propensity resulting from these behaviors. The present review summarizes the neurobiological background and correlational physiological and behavioral data in support of the involvement of skin temperature in the modulation of sleep propensity. It moreover points out the type of experimental investigations needed to support or refute the hypothesis. (C) Published by Elsevier Ltd.