Several daily measurements are necessary to reliably assess the cortisol rise after awakening: state- and trait components

The cortisol rise after awakening (CAR) is a frequently applied measure of pituitary-adrenal activity. This measure seems to reflect the acrophase of the diurnal cycle and can easily be assessed in saliva samples, collected by the proband or patient under real life conditions. Since different state and trait factors affect the CAR, we here address the questions (a) to which extent state and trait factors affect the CAR, and (b) how often cortisol measures after awakening have to be taken to obtain reliable results. In this study, we assessed the CAR on 6 consecutive days. After applying structural equation models and correlation analyses, we conclude that (a) the CAR of a single day is determined to a great extent by situational factors and only for a small proportion by trait factors and (b) from two (AUC(t)) to six (AUC(i)) days are necessary to achieve reliable trait measures, since state factors bias data from a single day