Hypothalamic homeothermy across the ultradian sleep cycle

In the ambient thermal zone for the vasomotor regulation of body temperature hypothalamic temperature changes across the states of the ultradian sleep cycle are the result of state-dependent heat production-heat loss imbalances affecting the temperature of the arterial blood perfusing the brain. However, the changes in arterial blood temperature are efficiently buffered, at a low energetic cost, by the thermal inertia of the mass of body water. Thus, the oscillations in hypothalamic temperature are maintained within a width of a few tenths of a degree and are so small as to be subliminal as thermal feedback stimuli for thermoregulatory responses. This passive hypothalamic homeothermy would support the hypothesis that a phylogenetic pressure was operative early on in mammals in order to limit the duration of the ultradian sleep cycle so as to fit the thermal inertia of the different masses of body water in mammals of different sizes.