MORE THAN A MARKER: INTERACTION BETWEEN THE CIRCADIAN REGULATION OF TEMPERATURE AND SLEEP, AGE-RELATED CHANGES, AND TREATMENT POSSIBILITIES

The neurobiological mechanisms of both sleep and circadianregulation have been unraveled partly in the last decades. A network of brainstructures, rather than a single locus, is involved in arousal state regulation,whereas the suprachiasmatic nucleus (SCN) has been recognized as a key structurefor the regulation of circadian rhythms. Although most models of sleep regulationinclude a circadian component, the actual mechanism by which the circadiantiming system promotes—in addition to homeostatic pressure—transitionsbetween sleep and wakefulness remains to be elucidated. Little more can bestated presently than a probable involvement of neuronal projections and neurohumoralfactors originating in the SCN. This paper reviews the relation among bodytemperature, arousal state, and the circadian timing system and proposes thatthe circadian temperature rhythm provides an additional signaling pathwayfor the circadian modulation of sleep and wakefulness. A review of the literatureshows that increased brain temperature is associated with a type of neuronalactivation typical of sleep in some structures (hypothalamus, basal forebrain),but typical of wakefulness in others (midbrain reticular formation, thalamus).Not only local temperature, but also skin temperature are related to the activationtype in these structures. Warming of the skin is associated with an activationtype typical of sleep in the midbrain reticular formation, hypothalamus, andcerebral cortex (CC). The decreasing part of the circadian rhythm in coretemperature is mainly determined by heat loss from the skin of the extremities,which is associated with strongly increased skin temperature. As such, alterationsin core and skin temperature over the day could modulate the neuronal activationstate or “preparedness for sleep” in arousal-related brain structures.Body temperature may thus provide a third signaling pathway, in addition tosynaptic and neurohumoral pathways, for the circadian modulation of sleep.A proposed model for the effects of body temperature on sleep appears to fitthe available data better than previous hypotheses on the relation betweentemperature and sleep. Moreover, when the effects of age-related thermoregulatoryalterations are introduced into the model, it provides an adequate descriptionof age-related changes in sleep, including shallow sleep and awakening closerto the nocturnal core temperature minimum. Finally, the model indicates thatappropriately timed direct (passive heating) or indirect (bright light, melatonin,physical activity) manipulation of the nocturnal profile of skin and coretemperature may be beneficial to disturbed sleep in the elderly. Althoughsuch procedures could be viewed by researchers as merely masking a markerfor the endogenous rhythm, they may in fact be crucial for sleep improvementin elderly subjects. (Chronobiology International,17(3), 313–354, 2000)

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