Concordance between self-reported and objective wakeup times in ambulatory salivary cortisol research.

BACKGROUND: Hypothalamic-pituitary-adrenal (HPA) axis functioning has implications for physical and mental health. One important indicator of HPA axis functioning, the salivary cortisol awakening response (CAR), is sensitive to whether participants provide their samples at the requested times after waking.
PURPOSE: To examine the extent to which adolescents report morning wakeup times accurately, test the impact of inaccurate waketime reporting on compliance with a salivary cortisol sampling protocol designed to estimate the CAR, and to examine the implications of non-compliance for CAR estimates.
METHOD: In a sample of 91 late adolescents, objective waketimes determined using actigraphy were compared to self-reported waketimes. Associations between accuracy of waketimes and compliance with requested morning cortisol sampling timings (wakeup and 30 min post-awakening) were examined, as were implications of non-compliance for the size of the CAR.
RESULTS: In terms of accuracy, 72% of self-reported waketimes were within 5 min and 90% were within 15 min of objective waketimes. Individuals who were more than 5 min discrepant in their waketime reporting, however, had a 90% decrease in their likelihood of being compliant-taking both morning cortisol samples within the requested time frames after waking. However, CARs were significantly lower only among individuals whose subjective and objective waketimes differed by more than 15 min.
CONCLUSIONS: Self-reported waketimes were reasonably accurate when compared to objective estimates of time of waking. When available, however, estimates of compliance are improved by knowledge of objective waketimes, resulting in increased accuracy of CAR estimates.