Twenty-four-hour cortisol response to multiple daily exercise sessions of moderate and high intensity

The aim of this study was to characterize and describe the cortisol respons es in athletes over a 24-h period on different days involving multiple exer cise sessions of varied intensity. Seventeen endurance athletes volunteered to undergo three experimental treatment conditions: (a) a control day invo lving no exercise; (b) an exercise day with two sessions of high-intensity exercise; and (c) an exercise day with two sessions of moderate-intensity e xercise. Significant changes (P<0.01) were found owing to the influence of both types of exercise during the daytime (high-intensity exercise producin g a greater cortisol response than moderate intensity). At night after the moderate-intensity exercise (at selected times), cortisol response was sign ificantly less than during the control condition at corresponding times (P< 0.05). After high intensity exercise (at selected times), cortisol levels w ere significantly less (P<0.05) than in the control condition at correspond ing times. Additionally, certain night-time responses after the high-intens ity exercise were significantly less than the moderate-intensity exercise r esponses at similar time paints. The results indicate that daytime multiple exercise sessions produce suppressed cortisol levels at night, and the mag nitude of this effect is dependent upon the intensity at which the daytime exercise is performed. The physiological mechanism inducing this effect upo n cortisol levels at night is unclear from the present data.