Nder the curve was divided into several trapezoids, and a total AUC was obtained as the sum of the areas of these individual trapezoids (Yeh and Kwan, 1978). Time, on the x-axis, was measured in minutes since wake-up. AUC was calculated for the period between wake-up and 16 hours after wakeup to ensure that each participant contributed the same HMPL-012 mechanism of action number of waking hours to the measure. Cortisol values 1.64028E+14 for 16 hours were linearly interpolated based on adjacent values. Because it is a summary measure, cortisol values were not log transformed when calculating the AUC, which is in units of nmol/L*minutes. Mixed models with random intercepts (to account for within person correlations in the three daily measures) were used to model AUC as a function of SES and race/ethnicity adjusted for covariates. All times used in analyses were those registered j.addbeh.2012.10.012 by the Track-Caps device. Since participants were instructed to take their first sample when they woke up, the time of the first sample was used as the wake-up time. For the small number of days for which no first sample was collected, but at least one of the other samples was, the wake-up time recorded on the daily questionnaire was used instead (40/2899, 1.4 ). Days missing both the first sample and the reported wakeup time (5/2899, 0.2 ) were excluded. The 1002 participants enrolled in the MESA Stress Study yielded a maximum of 3006 participant-days of data collection. Of these 127 days were excluded because no track-caps times were available (n=107 days), no cortisol samples were collected (n=15 days), or because there was no first cap time or reported wake-up time to use for a wake-up time (n=five days). We excluded 936 samples with no track-cap time, insufficient sample for assay, or unreliable cortisol value (0 or >100 nmol/L). Lastly we excluded those that reported taking oral or inhaled steroids (n=35 persons). This resulted in a total of 935 participants, 2774 days, and 15774 samples for analysis.NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript ResultsTable 1 shows selected characteristics of study participants by site, age, sex, race/ethnicity and SES indicators. The median age of the participants was 65 years. Approximately 49 of the sample was male, 20 were white, 28 black, and 53 Hispanic. Approximately 85 of participants collected at least five samples per day for all days on which they collected samples (97 of participants collected samples on all three days). The percentage of participants with at least five samples per day was similar across socio-demographic characteristics. Overall 86 of self-recorded times were within 15 minutes of the registered Track-Cap times a measure of time-recording accuracy. Younger participants, whites, and participants with higher SES showed higher percentages of time recording accuracy (p <0.001 for all comparisons). Overall the first sample was taken within five minutes of wake-up for 78 of days across participants. Again this measure of concordance was higher among younger people, whites and participants with higher SES. The median wake-up time (first sample) was 0642h and the median bedtime (last sample) was 2226h.