Developmental changes in baseline cortisol activity in early childhood: relations with napping and effortful control.

Development of the hypothalamic-pituitary-adrenocortical (HPA) axis was examined using salivary cortisol levels assessed at wake-up, midmorning, midafternoon, and bedtime in 77 children aged 12, 18, 24, 30, and 36 months, in a cross-sectional design. Hierarchical linear modeling (HLM) analyses were used to characterize cortisol production across the day and to examine age-related differences. Using area(s) under the curve (AUC), cortisol levels were higher among the 12-, 18-, and 24-month children than among the 30- and 36-month children. For all five age groups, cortisol levels were highest at wake-up and lowest at bedtime. Significant decreases were noted between wake-up and midmorning, and between midafternoon and bedtime. Unlike adults, midafternoon cortisol levels were not significantly lower than midmorning levels. Over this age period, children napped less and scored increasingly higher on parent reports of effortful control. Among the 30- and 36-month children, shorter naps were associated with more adultlike decreases in cortisol levels from midmorning to midafternoon. Considering all of the age groups together, effortful control correlated negatively with cortisol levels after controlling for age. These results suggest that circadian regulation of the HPA axis continues to mature into the third year in humans, and that its maturation corresponds to aspects of behavioral development.