EFFECTS OF EXERCISE ON NEUROENDOCRINE SECRETIONS AND GLUCOSE REGULATION AT DIFFERENT TIMES OF DAY

To study the effects of time of day on neuroendocrine and metabolic re sponses to exercise, body temperature, plasma glucose, insulin secreti on rates (ISR), and plasma cortisol, growth hormone (GH) and thyrotrop in (TSH) were measured in young men, both at bed rest and during a 3-h exercise period (40-60% maximal O-2 uptake). Exercise was performed a t three times of day characterized by marked differences in cortisol l evels, i.e., early morning (n = 5), afternoon (n = 8), and around midn ight (n = 9). The subjects were kept awake and fasted, but they receiv ed a constant glucose infusion to avoid hypoglycemia. Exercise-induced elevations of temperature were higher in the early morning than at ot her times of day. The exercise-induced glucose decrease was similar to 50% greater around midnight, when cortisol was minimal and not stimul ated by exercise, than in the afternoon or early morning (P < 0.05). T his effect of time of day appeared unrelated to decreases in ISR or in creases in temperature and GH. Robust TSH increases occurred in all ex ercise periods and were maximal at night. The results demonstrate the existence of circadian variations in neuroendocrine and metabolic resp onses to exercise.