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Stress research in youth typically considers either the autonomic nervous system or HPA axis. However, these systems are highly coordinated and physically interconnected. We examined whether the inter-relation between cardio-autonomic and HPA axis measures was better associated with perceived stress than their singular associations. Children and adolescents ( N = 201) collected saliva samples to measure cortisol (AUC AG , AUC I , maximum), wore an electrocardiogram monitor for 24 h to derive heart rate variability (HRV; LF, HF, LF/HF ratio), and completed the Perceived Stress Scale. The interaction between sympathovagal modulation (LF, LF/HF ratio) and cortisol awakening response (AUC AG , AUC I , maximum) explained significantly greater variance in perceived stress than either stress system alone. Higher sympathovagal modulation combined with higher cortisol awakening response was associated with greater perceived stress. Findings suggest that the inter-relation between cardio-autonomic and HPA axis activity may advance our understanding of how stress impacts health. [ABSTRACT FROM AUTHOR]

Summary: The diurnal cortisol profile has been implicated in multiple physical and mental health conditions in children and adolescents; however, current knowledge regarding the stability of the diurnal cortisol profile is largely based on adults. Developmental changes throughout childhood and adolescence warrant examination of the stability of the diurnal cortisol profile during this stage in the lifecourse. The aim of the present study was to conduct a comprehensive evaluation of the diurnal cortisol profile in children and adolescents. Participants (N =233; M =12.40, SD =1.83; 44.2% girls) in the Healthy Heart Project collected saliva samples, completed demographic questionnaires, and recorded bed and waking time. Intra-class correlations were calculated to evaluate the stability of aggregate and single sample measures of the diurnal cortisol profile. Total cortisol concentration (AUCTG, AUCAG) and maximum sample were the most stable cortisol measures (ICCavg =0.54). Dynamic measures (AUCI, slope; ICCavg =0.22) and other single sample measures (awake, lunch, dinner, bedtime, morning random, day random; ICCavg =0.28) were less stable. Of the developmentally relevant covariates tested, sleep duration, adrenarche, and time of awakening were most associated with cortisol values. Altogether, the diurnal cortisol profile yielded moderate to high stability in children and adolescents. These findings can inform methodological decisions regarding cortisol sampling protocols for children and adolescents. [ABSTRACT FROM AUTHOR]