Circadian Body Temperature Rhythm and the Interaction with Energy State

We have revealed that circadian body temperature (T b ) rhythm is significantly influenced by fasting/fasting-related hormones. The effect of circadian mechanism and fasting/fast -ing-related hormones on thermoregulation was examined. Fasting decreases T b during the light phase in rodents. For the regulation, the suprachiasmatic nucleus (SCN) and clock genes, such as Cry and Clock , are necessary. In addition, ghrelin and several hypothalamic nuclei, that is, the medial preoptic area, paraventricular nucleus (PVN), and arcuate nucleus (ARC), play a key role in the T b rhythm. During the light phase, fasting and ghrelin affect the hypothalamic areas. The activity of the SCN increases and that of the ARC decreases. The SCN sends inhibitory signals to the PVN, which may result in a lower heat production in the interscapular brown adipose tissue (iBAT) and T b . By contrast, during the dark phase, the activity of the SCN decreases and that of the ARC increases. The inhibitory signal from the SCN is less, and the PVN is activated. Heat production of the iBAT increases and T b is maintained. There are functional and anatomical connections between the circadian and thermoregulation systems. The circadian system modulates thermoregulatory response to hypothermia and/or cold depending on time and feeding condition. same peak phases during the LD conditions. As for the T b and V ˙ O 2 rhythms in the LD condition, the daily means were lower, and the amplitudes of the rhythms were higher in the food-restriction condition than those under the ad lib feeding condition. The study showed that the circadian T b rhythm is observed even in mice that lack the internal circadian mechanism, when an external lighting and feeding stimuli that alter heat production are observed. The result showed that the heat production rhythm may be a key component for the T b rhythm.