Background: Studies investigating the association between diurnal cortisol rhythm and behavioural problems in young children have yielded inconsistent results. We tested the hypothesis that variations in diurnal cortisol rhythm in pre-schoolers are already related to problem behaviour early in life with a cross-sectional and longitudinal design. Methods: This study was embedded in Generation R, a population-based cohort from foetal life onwards. Parents collected saliva samples from their infant at 5 moments during 1 day. In 322 infants aged 12-20 months, we determined the diurnal cortisol rhythm by calculating the area under the curve (AUC), the cortisol awakening response (CAR), and the diurnal slope. Problem behaviour was assessed at ages 1.5 and 3 years with the Child Behavior Checklist/1.5-5 years. Results: No cross-sectional associations between the cortisol composite measures and problem behaviour were found at 1.5 years. However, cortisol predicted change in internalizing problems as assessed from 1.5 to 3 years, but not change in externalizing problems. Children with higher AUC levels, flatter slopes and a more positive CAR at baseline were more likely to score higher on the Internalizing Problems scale (β per nmol/L AUC: 0.08, 95% CI: 0.00; 0.17, p=. 0.04; β per nmol/L/h slope: 0.57, 95% CI: 0.17; 0.98, p=. 0.006; β per nmol/L CAR: 0.04, 95% CI: 0.01; 0.08, p=. 0.02) at follow-up. Conclusions: Variations in diurnal cortisol rhythm are associated with change in internalizing problems in pre-schoolers. The results suggest that variations in diurnal cortisol patterns early in life precede internalizing problems.

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Keywords Cohort, Cortisol, Diurnal rhythm, HPA axis, Infants, Longitudinal, Problem behaviour
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Journal Psychoneuroendocrinology
Saridjan, N.S, Velders, F.P, Jaddoe, V.W.V, Hofman, A, Verhulst, F.C, & Tiemeier, H.W. (2014). The longitudinal association of the diurnal cortisol rhythm with internalizing and externalizing problems in pre-schoolers. The Generation R Study. Psychoneuroendocrinology, 50, 118–129. doi:10.1016/j.psyneuen.2014.08.008