Daily activity and body temperature.

Body temperature varies between 36 and 39 degrees C in states ranging from sleep to high levels of sustained exercise, but it is not known whether this continuum of body temperature is related to a continuum of activity. Calorimetric studies of sedentary days were undertaken with four levels of food intake, men doing mild sustained exercise, and men and women walking and cycling vigorously. Steady states of metabolism were followed by slow exponential changes to steady states of heat loss (sigma Q), followed in turn by changes in rectal temperature (T(re)). Regression analysis showed a continuous, curvilinear relationship between sigma Q and T(re) from the low end of the activity spectrum (50 W) to progressively higher levels of exercise (600 W). These related continua of activity and body temperature appear to be the result of heat regulation.