CIRCADIAN-RHYTHM OF CORTISOL CONFOUNDS CORTISOL RESPONSES TO EXERCISE- IMPLICATIONS FOR FUTURE-RESEARCH

To investigate whether measurements of cortisol responses to exercise are confounded by neglect of the hormone's circadian rhythm, we measur ed the serum and salivary cortisol responses of eight women to 40 min of 70% maximal oxygen consumption treadmill exercise beginning at 0800 and 2000. Responses were calculated relative to the usually employed preexercise concentrations and also to concentrations at the same time s of another day while subjects were at rest. Compared with areas unde r response curves (AUCs) calculated relative to their circadian baseli nes, AUCs for serum and salivary cortisol calculated by reference to p reexercise concentrations were underestimated (serum, P < 0.001; saliv ary, P < 0.01) by 93 and 84% in the morning and by 37 and 35% in the e vening, respectively. Calculated by the usual preexercise baseline met hod, rises in serum and salivary cortisol were similarly underestimate d. More accurately calculated relative to their circadian baselines, s erum and salivary cortisol AUCs were similar (P = 0.63 and P = 0.37, r espectively) in the morning and evening, as were their rises (P = 0.23 and P = 0.70, respectively). In future investigations of the existenc e and magnitude of cortisol responses, those responses must be calcula ted relative to the hormone's circadian baseline.