HYPOTHALAMIC HOMEOTHERMY ACROSS THE ULTRADIAN SLEEP CYCLE

In the ambient thermal zone for the vasomotor regulation of body tempe rature hypothalamic temperature changes across the states of the ultra dian sleep cycle are the result of state-dependent heat production-hea t loss imbalances affecting the temperature of the arterial blood perf using the brain. However, the changes in arterial blood temperature ar e efficiently buffered, at a low energetic cost, by the thermal inerti a of the mass of body water. Thus, the oscillations in hypothalamic te mperature are maintained within a width of a few tenths of a degree an d are so small as to be subliminal as thermal feedback stimuli for the rmoregulatory responses. This passive hypothalamic homeothermy would s upport the hypothesis that a phylogenetic pressure was operative early on in mammals in order to limit the duration of the ultradian sleep c ycle so as to fit the thermal inertia of the different masses of body water in mammals of different sizes.