Cortisol and growth hormone responses to exercise at different times of day

Exercise of appropriate intensity is a potent stimulus for GH and cortisol secretion. Circadian and diurnal rhythms may modulate the GH and cortisol r esponses to exercise, but nutrition, sleep, prior exercise patterns, and bo dy composition are potentially confounding factors. To determine the influe nce of the time of day on the GH and cortisol response to acute exercise, w e studied 10 moderately trained young men (24.1 +/- 1.1 yr old; maximal oxy gen consumption, 47.9 +/- 1.4 mL/kg.min; percent body fat, 13.2 +/- 0.6%). After a supervised night of sleep and a standard meal 12 h before exercise, subjects exercised at a constant velocity (to elicit an initial blood lact ate concentration of similar to2.5 mmol/L) on a treadmill for 30 min on 3 s eparate occasions, starting at 0700, 1900, and 2400 h. Blood samples were o btained at 5-min intervals for 1 h before and 5 h after the start of exerci se; subjects were not allowed to sleep during this period. Subjects were al so studied on 3 control days under identical conditions without exercise. T here were no significant differences with time of day in the mean blood lac tate and submaximal oxygen consumption values during exercise. The differen ces over time in serum GH and cortisol concentrations between the exercise day and the control day were determined with 95% confidence limits for each time of day. Exercise stimulated a significant increase in serum GH concen trations over control day values for approximately 105-145 min (P < 0.05) w ith no significant difference in the magnitude of this response by time of day. The increase in serum GH concentrations with exercise was followed by a transient suppression of GH release (for <similar to>55-90 min; P ( 0.05) after exercise at 0700 and 1900 h, but not at 2400 h. Although the duratio n of the increase in serum cortisol concentrations after exercise was simil ar (similar to 150-155 min; P < 0.05) at 0700, 1900, and 2400 h, the magnit ude of this increase over control day levels was greatest at 2400 h. This d ifference was significant for approximately 130 min and approximately 40 mi n compared to exercise at 1900 and 0700 h, respectively (P < 0.05). The cor tisol response to exercise at 0700 h was significantly greater than that at 1900 h for about 55 min (P < 0.05). A rebound suppression of cortisol rele ase for about 50 min (P < 0.05) was observed after exercise at 2400 h, but not 0700 or 1900 h. Both baseline (before exercise) and peak cortisol conce ntrations were significantly higher at 0700 h than at 1900 or 2400 h (P < 0 .01). We conclude that time of day does not alter the GH response to exerci se; however, the exercise-induced cortisol response is modulated by time of day.