Daytime trajectories of cortisol: demographic and socioeconomic differences--findings from the National Study of Daily Experiences.

Cortisol's daytime rhythm is thought to be altered by aging and by exposure to chronic stress. However, measurement of an individual's usual cortisol rhythm is hampered by the effects of acute stressors, by differences between working days and weekends, by between-day variation in waking time and sleep duration, by variability in cortisol sampling times, and by possible variability in the timing of cortisol peak and nadir. Therefore, to determine differences in the usual daytime cortisol rhythm by age, socioeconomic status, and race/ethnicity, we measured salivary cortisol levels at four time-points, repeated over four days that included both weekdays and weekend days, in 1693 men and women from a national sample, and used three alternate growth curve specifications for the underlying cortisol rhythm (linear spline, quadratic spline, piece-wise linear-cubic) in order to minimize the impact of sample timing and other methodological issues. Model-predicted mean values of (and demographic and socioeconomic differences in) cortisol peak, nadir, and area under the curve (AUC) were nearly identical across model specifications. Older age and male gender were independently associated with higher cortisol peak, nadir, and AUC. Low education and minority race/ethnicity status were independently associated with lower cortisol peak and higher nadir, but were not associated with AUC. We also found significant cortisol peak and AUC associations with waking time, sleep duration, and workday vs. weekend day status, suggesting the importance of measuring these confounders and of collecting cortisol measurements over multiple days in research studies. We conclude that daytime cortisol levels are higher in older age and in men compared to women, and that the daytime cortisol rhythm is flatter (more blunted) in less privileged segments of society. Flattening of daytime cortisol rhythms may represent one mechanism by which social stressors lead to poor health outcomes.