Circadian rhythmicity and homeostatic stability in thermoregulation

Stability and circadian variation in core body temperature (Tc) were believed to be homeostatic responses until well into the 20th century. Defense of a narrow thermoneutral range was well documented, whereas circadian oscillations were attributed to episodic biochemical and environmental stimuli or chronological stressors in life routines. Research in thermal physiology has illuminated several of the "black boxes" in the understanding of temperature regulation, and advances in chronobiology have shattered old paradigms. While these discoveries are still evolving, existing information provides valuable clues about physiological responses to heat loss or over-heating that could improve clinical assessment and intervention. Discoveries that circadian rhythm of Tc is regulated by an endogenous "clock" and is remarkably stable have helped to make it the most widely used circadian indicator. More recently, Tc was found to exert its own cyclic rhythm under free-running conditions. While some investigators claim that circadian and homeostatic processes are independent, there are conditions in which clinical distinctions are less clear. This overview reviews contemporary scientific findings about circadian and homeostatic processes in thermoregulation. Examples are drawn from human and animal research. Physiological responses and mechanisms are explained in relation to their relevance to clinical treatment or health care. Gaps in existing research and application are discussed.