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This study examined salivary cortisol, a stress-sensitive hypothalamic-pituitary-adrenocortical (HPA) axis hormone in 20 infants (12 females; M age = 10.8 months) and 35 toddlers (20 females; M age = 29.7 months) in full-day, center-based child care. Samples were taken at approximately 10:00 a.m. and 4:00 p.m. at child care and at home. At child care, 35% of infants and 71% of toddlers showed a rise in cortisol across the day; at home, 71% of infants and 64% of toddlers showed decreases. Toddlers who played more with peers exhibited lower cortisol. Controlling age, teacher-reported social fearfulness predicted higher afternoon cortisol and larger cortisol increases across the day at child care. This phenomenon may indicate context-specific activation of the HPA axis early in life.

In this study, we examined a model that describes both direct and indirect pathways between children's temperament and activity of the hypothalamic-pituitary-adrenal (HPA) axis when children are in peer-group settings. We hypothesize that in peer-group settings both shy, inhibited and exuberant, undercontrolled children would exhibit higher cortisol levels, but these associations would operate through different pathways. Sociometric measures of peer rejection, salivary cortisol, and teacher reports of temperament were collected on 82 preschoolers. Children who were rejected by classmates had higher cortisol levels than the other children. The combination of Surgency and Poor Effortful Control (Effortful Control, reverse scored) was associated with elevated cortisol through a pathway mediated by aggressive interactions with peers and peer rejection. With the indirect path explained, the combination of Surgency and Poor Effortful Control also was directly and negatively associated with classroom cortisol levels. These results help explain why temperament associations with HPA activity have been variable and difficult to discern when children are assessed in peer-group contexts. In these contexts, both direct and indirect pathways between temperament and cortisol need to be examined.

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.

The hypothalamic-pituitary-adrenal (HPA) axis is influenced by early life adversity; however, less is known about the potential for recovery following marked improvements in care. The present study examined longitudinal changes in children’s cortisol reactivity in the laboratory (4 assessments over 2 years) after adoption. Post-institutionalized (N=65) and post-foster care children (N=49) demonstrated blunted reactivity relative to non-adopted peers (N=53). Furthermore, post-institutionalized children exhibited no evidence of expected adaptation to repeated sessions in the 2 years following adoption. As evidenced by blunted cortisol reactivity, flatter diurnal slope, and lower home morning cortisol, we found support for hypocortisolism among children experiencing adverse early care. Hypocortisolism served as a mediator between adversity and teacher-reported attention and externalizing problems during kindergarten. Early adversity appears to contribute to the down-regulation of the HPA axis under both basal and stress conditions.

Diurnal salivary cortisol profiles are valuable indicators of adrenocortical functioning in epidemiological research and clinical practice. However, normative reference values derived from a large number of participants and across a wide age range are still missing. To fill this gap, data were compiled from 15 independently conducted field studies with a total of 104,623 salivary cortisol samples obtained from 18,698 unselected individuals (mean age: 48.3 years, age range: 0.5 to 98.5 years, 39% females). Besides providing a descriptive analysis of the complete dataset, we also performed mixed-effects growth curve modeling of diurnal salivary cortisol (i.e., 1 to 16 hours after awakening). Cortisol decreased significantly across the day and was influenced by both, age and sex. Intriguingly, we also found a pronounced impact of sampling season with elevated diurnal cortisol in spring and decreased levels in autumn. However, the majority of variance was accounted for by between-participant and between-study variance components. Based on these analyses, reference ranges (LC/MS-MS calibrated) for cortisol concentrations in saliva were derived for different times across the day, with more specific reference ranges generated for males and females in different age categories. This integrative summary provides important reference values on salivary cortisol to aid basic scientists and clinicians in interpreting deviations from the normal diurnal cycle.

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