Use of a resting control day in measuring the cortisol response to mental stress: diurnal patterns, time of day, and gender effects.

In laboratory studies of individual differences in stress reactivity, cortisol responses are typically measured by comparing a prestress baseline with values obtained at the end of the stressor. In the present study, we measured cortisol in this manner on a stress day, but we also took samples on a second day when the volunteers rested in the lab at the same time of day and for the same duration. We compared stress responses as the difference from pre- to poststress and also poststress vs. rest day control. The latter method allowed a greater appreciation of how stress perturbed the underlying diurnal baseline. Although the effect of stress was statistically significant when measured as the change from pre- to poststress, the magnitude of the effect was 54% larger when measured against the time-of-day control from the rest day. In particular the diurnal control method provided a wider range of stress values that potentially provide a greater range of response values in carrying out analyses of individual differences. Published by Elsevier Ltd.