Morning and Afternoon Bouts of Exercise Result in the Same Physiological Stress Response

Abstract

Circulating cortisol levels increase in response to physiological stress beyond levels observed in normal diurnal patterns. To compare the physiological stress response after a signal bout of exercise in the morning versus the afternoon, participants were assigned to three groups: morning exercise (n=8), afternoon exercise (n=7), or no exercise (n=7). Morning and afternoon exercise groups participated in 15 minutes of training, alternating resistance and cardiovascular activities, at 07:00 or 15:00 hours, respectively. Salivary cortisol samples were taken from all participants throughout the day. Pulse and blood pressure were recorded to ensure consistency of exertion during exercise bouts. Mean peak cortisol concentrations following exercise occurred 30–45 minutes after completion of exercise (1.16 ± 0.237 μg/dL and 0.651 ± 0.177 μg/dL, morning and afternoon exercise groups, respectively). There was a significant increase in salivary cortisol levels, as indicated by increase in area under the curve (AUC), between the no exercise and the morning exercise groups (p=0.0009), with cortisol levels reaching values almost twice the magnitude in the morning exercise group. Time of day did not affect the physiological stress response to exercise, as there was no significant difference in salivary cortisol AUC. V. Dautermann and N. Mantina were supported by the Beloit College Biomedical Scholars Program.