Modelling the relation of body temperature and sleep: importance of the circadian rhythm in skin temperature

A close relation between thermoregulatory and arousal related processes in basal forebrain and hypothalamic areas has been substantiated in many studies. It has been demonstrated especially-but not exclusively-in the preoptic area of the anterior hypothalamus (POAH), that a subpopulation of warm-sensitive neurons (WSNs) spontaneously increases firing rate at sleep onset, and that experimental local warming of the POAH induces a similar increase in firing rate and facilitates sleep onset. It has consequently been proposed that brain temperature may be involved in physiological regulation of sleep 4. However , contrary to the experimental findings of increased sleep onset probability with a locally increased hypothalamic temperature, the likeliness of sleep onset in unmanipulated conditions is actually minimal at the time when the cir-cadian rhythm in temperature reaches its peak. In fact, sleep onset probability increases on the falling limb of the circadian core temperature rhythm. How can this discrepancy of increased sleep with well-controlled experimental local POAH warming be reconciled with increased sleep with local cooling under natural conditions? A suitable explanation has been put forward 5 : in modelling based on local warming experiments, it has generally been ignored that many of the locally thermosensitive neurons also respond, in a similar way, to changes in skin temperature. Thus, the very changes in cell membrane properties leading to sleep-related alterations in firing rate as induced by local warming in experimental conditions, may be induced by warming of the skin under natural conditions. Several considerations support the importance of skin temperature. First, after warming of the skin, preoptic WSNs show a markedly increased firing rate to a level that can be reached only by extreme (non-physiological) local warming. Moreover, with elevated skin temperature, WSNs have a high firing rate irrespective of the local temperature, i.e. temperature input from the skin appears to dominate when competing signals are present 1, 2. Second, sleep appetitive behavior like lying down, covering etc. is associated with a redistribution of warm blood to the extremities, thus increasing their temperature. Third, and 154 Figure: The upper panel shows the average 24-hour profile of human rectal temperature and temperature at the extremities under constant routine conditions 3. For convenience , the curves have been moved freely on the vertical axis, omitting absolute values on the vertical axis, while the relative magnitude of the amplitudes is preserved in the figure. Both curves affect the probability of sleep-type neuronal firing patterns of …