How is the circadian rhythm of core body temperature regulated?

How is the circadian rhythm of core body temperature regulated? The circadian rhythm of core body temperature (CBT) is a well-documented physiological phenomenon. Already in 1842, Gierse [6] had shown that his own oral temperature revealed a maximum temperature in the early evening and a minimum in the early morning hours with a maximum-minimum range of 0.9 °C. It had been assumed for a long time that muscular activity (exercise) and digestive processes were the most important factors for generation of the CBT rhythm [8]. Aschoff and his colleagues systematically explored the causes of this rhythm [1, 2]. He showed that the circadian rhythm of CBT is determined both by changes in heat production and changes in heat loss, and concluded that heat production undergoes a circadian rhythm which is phase advanced by 1.2h with respect to the circadian rhythm of heat loss, i. e. when heat production surpasses heat loss, CBT increases – transport of heat needs time. Therefore, when we want to explain changes in CBT we need to know the relationship between heat production and heat loss. Under resting conditions, heat production depends mainly on the metabolic activity of inner organs such as liver, intestines, kidneys, heart in the abdominal/thoracic cavity, and the brain, together producing ca. 70 % of the entire resting metabolic rate of the human body [3]. However, this " core " heat is generated in only 8 % of the body mass with a surrounding skin surface of only about 0.3m 2 (surface to volume = 0.1) [3]. The proximal skin surface is not ideally shaped: too flat for good heat transfer to the environment. This means, that even in a comfortable thermoneutral environment, heat has to be transferred from the core to parts of the body with better heat transfer capacities, namely to the extremities (surface to volume coefficient e. g. of fingers = 2.2) [3]. These distal parts of the body have ideal (round) surface shapes for good heat transfer properties to the environment. Blood is the main medium for transporting heat from the core to distal skin regions (convectively), driven and distributed by the cardiovas-cular system. The human body consists therefore of two compartments, the heat producing core, and the heat-loss regulating shell [3]. CBT (especially in the brain) is homeostatically regulated, and the shell is poikilother-mic and therefore largely dependent on environmental conditions. The shell serves as a kind …