Body Temperature

The daily oscillation of body temperature in mammals constitutes one of the most regular and predictable of mammalian circadian rhythms. Historically, the body temperature rhythm is of unique importance in the circadian field. In addition, the Tb rhythm is also easily measured, coupled to numerous other body rhythms, and stable. Whereas the Tb rhythm integrates many other aspects of physiology, for the most part it is generated endogenously. The overt or expressed rhythm is a complex product of physiological integration among many control systems, including the thermoregulatory, cardiovascular, sleep-wake, and circadian systems. In addition, the endogenous temperature rhythm can be modified by factors such as activity, feeding, ambient temperature, and light. However, in a controlled environment, the Tb rhythm can provide an extremely accurate picture of the underlying clock activity.

The influence of the CTS on body temperature homeostasis is well-documented. The daily rhythm of body temperature is generated by the relative phasing of the heat production and heat loss rhythms in both entrained and free-running subjects. Changes in the ambient temperature produce coordinated and compensatory changes in heat production and heat loss; however, their phase relationships with the Tb rhythm are conserved. As with all circadian rhythms, the temperature rhythm is controlled by a circadian oscillator. Interestingly, this circadian oscillator may reside in another nucleus separate from the SCN. Some studies support the hypothesis that the thermoregula-tory system is controlled by two or more circadian oscillators. The concept of multioscillator control of body temperature is old and is derived, in part, from studies in which the body temperature rhythm persisted following SCN lesions. However, other studies of similarly placed lesions have evidenced a highly attenuated or absent body temperature rhythm. These findings suggest that a second (or more) oscillator would likely be found very close to the SCN, perhaps in the hypothalamic preoptic area or subparaventricular zone.

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