Morningness-eveningness, habitual sleep-wake variables and cortisol level

The free cortisol rise immediately after awakening is a distinctive part of the cortisol circadian rhythm. Few studies have focused on the relationship between morningness and cortisol awakening response (CAR). As the suprachiasmatic nucleus may be of major importance for both, one may hypothesise that the CAR should also be associated with morningness. We used saliva samplings and applied questionnaires in adolescents and young adults. Saliva samplings were made at awakening (t1) and 30 min later (t2). Chronotype was measured using the Composite Scale of Morningness (CSM), and habitual time in bed was calculated from habitual rise times and bed times. In both, the adolescent sample and the young adults, positive correlations existed between cortisol measures at t1 immediately after awakening and CSM scores. Using a multivariate model controlling for age group, smoking status and time of t1, we identified CSM scores and bed times as the most relevant variables for CAR. When comparing extreme chronotypes, morning types had higher cortisol levels immediately after awakening than evening types. These results strongly suggest an association between morningness-eveningness and CAR.