Abstract

Cortisol is the end product of the hypothalamic pituitary adrenal axis (HPA). It is released in a diurnal profile with a noticeable morning rise (cortisol awakening response) and decline throughout the day (diurnal slope). Cortisol is of interest to many researchers due to its association with negative physical and mental health consequences. For the diurnal cortisol profile to be considered a stable individual difference, it must be reliable to measure. Current knowledge of the reliability of the diurnal cortisol profile is almost entirely based on adults. The reliability in youth may differ due developmental factors, such as puberty, among other possible covariates. The present study evaluated the reliability of calculated indices and individual measures of the diurnal cortisol profile in youth aged 9 to 18 years. Three groups of youth collected five to six saliva samples per day over two to three days. Cortisol assays and calculated indices were conducted using standardized methods. Results indicated maximum peak cortisol level, the total cortisol concentration over a day (AUC TG ), and the cortisol awakening response relative to ground (AUCAG ) can be moderately reliably assessed in children and adolescents when sampled over two to three days. At least seven days are needed to obtain reliable measures of the change in cortisol concentration (AUCI , diurnal slope). Important covariates to consider include sleep duration, day of week, pubertal stage, time of awakening, and perceived stress. These findings suggest the diurnal cortisol profile in children and adolescents can be reliably assessed and reflects a stable individual difference. Methodological considerations and suggestions for future research are discussed.