Further support for higher salivary cortisol levels in "morning" compared to "evening" persons

We analyzed free cortisol daytime levels in a "3-days-7-samples-a-day" protocol in relation to morningness in 112 middle-aged male and female adults derived from the first year of data of the Trier Teacher Stress Study using a one-item proxy for the assessment of chronotype. First, log-transformed cortisol levels were analyzed by a repeated-measures approach controlling for relevant covariates. Results yielded an almost significant main effect of morningness (P=.06). Subsequent within-day analyses, additionally controlling for awakening time, resulted in (marginally) significant main effects of morningness at Day 1 (P=.06) and Day 3 (P=.05) and a significant interaction of sample-by-morningness at Day 2 (P=.04). In sum, the present data seem to corroborate the idea of higher daytime cortisol levels in morning relative to evening types in a sample of healthy middle-aged male and female adults.