Gravity and thermoregulation: metabolic changes and circadian rhythms

Gravity appears to alter thermoregulation through changes in both the regul ated level of body temperature and the rhythmic organization of temperature regulation. Gravity has been hypothesized to have an associated metabolic cost. increased resting energy expenditure and dietary intake have been obs erved in animals during centrifuge experiments at hypergravity. Thus far, o nly animals have shown a corresponding reduction in metabolism in micrograv ity. Altered heat loss has been proposed as a response to altered gravitati onal environments, but remains documented only as changes in skin temperatu re. Changes in circadian timing, including the body temperature rhythm, hav e been shown in both hypergravity and microgravity, and probably contribute to alterations in sleep and performance. Changes in body temperature regul ation may result from circadian disturbance, from the direct or indirect ac tions of gravity on the regulated temperature, or from changes in thermoreg ulatory effectors (heat production and heat loss) due to altered gravitatio nal load and convective changes. To date, however, we have little data on t he underlying thermoregulatory changes in altered gravity, and thus the pre cise mechanisms by which gravity alters temperature regulation remain large ly unknown.