Diurnal cortisol secretion at home and in child care: a prospective study of 2-year-old toddlers Isabelle Ouellet-Morin, 1 Richard E. Tremblay, 1,2,3,4 Michel Boivin, 1,5 Michael Meaney, 6 Michael Kramer, 7 and Sylvana M. Co ˆte ´ 2,8 1 Research Unit on Children’s Psychosocial Maladjustment, Montre ´al, Canada; 2 International Laboratory for Child and Adolescent Mental Health Development, INSERM U669, Paris, France; 3 Department of Pediatrics, Psychiatry and Psychology, Universite ´ de Montre ´al, Montre ´al, Canada; 4 School of Public Health and Population Science, University College Dublin, Ireland; 5 School of Psychology, Universite ´ Laval, Que ´bec city, Canada; 6 Douglas Hospital Research Centre, McGill University, Montre ´al, Canada; 7 The Research Institute of the McGill University Health Centre, Departments of Paediatrics and of Epidemiology, Biostatistics and Occupational Health, McGill University, Montre ´al, Canada; 8 Department of Social and Preventive Medicine, Universite ´ de Montre ´al, Montre ´al, Canada Background: Previous studies indicate that children may experience disrupted cortisol secretion in child care. The extent to which this is a transient or long-term disruption is not known, as most studies have relied on cross-sectional designs, and age-heterogeneous small sample sizes. This study aims to (a) compare cortisol secretion measured at home and in child care at 2 and 3 years of age, (b) investigate cortisol changes from 2 to 3 years of age, (c) examine whether age at initiation of child care is associated with cortisol secretion, and (d) investigate whether cortisol secretion in child care is linked to behavioural problems. Methods: Saliva samples were collected in a cohort of children recruited at 2 years of age from a larger population sample composed of women seen for the first time during pregnancy. Saliva was sampled twice a day (morning and afternoon) over two consecutive days at home and in child care at 2 (n = 155) and 3 years of age (n = 116). Interviews regarding the familial socio- economic background and child care history were conducted with the mothers. Results: At 2 years of age, children showed a flat diurnal cortisol pattern in child care and a decreasing pattern at home. At age 3 years, children showed decreasing patterns both at home and in child care. Also at 3 years, children with less child care experience (i.e., entry after 16 months) had higher cortisol levels in child care and lower levels at home. In contrast, those with more experience (i.e., entry prior to 8 months) had lower cortisol in child care and higher cortisol at home. Conclusion: The different patterns of diur- nal secretion observed in child care as compared to home is transient for most children, diminishing as they get older, whereas home and child care overall levels later on may be influenced by the cumulated experience with child care. Keywords: Toddlers, children, cortisol, stress, child care. Abbreviation: HPA: hypothalamic-pituitary-adrenal. Child care has become a common experience in western industrialised societies (Shpancer, 2002). More than half of children attend child care on a regular basis before the end of their preschool years (NICHD, 2005). Yet the long-standing debate on the impact of child care on children’s mental health continues (Belsky, 2002; NICHD, 2003). The idea that child care may be detrimental to children’s health has received some support from a study reporting higher cortisol levels in child care in comparison to home (Watamura, Donzella, Alwin, & Gunnar, 2003). Cortisol is a hormone produced by the hypothalamic-pituitary-adrenal (HPA) axis at resting states and in response to stress. The diurnal rhythm of cortisol is normally characterised by a peak reached shortly after awakening and decreas- ing levels during the rest of the day, a pattern established in humans within the first months of life (Custodio et al., 2007). Child care may represent a stressful challenge for young children as they are exposed to new physical settings, unknown peers and adults for extended periods of time in absence of their parents. Prior work has indeed shown that cortisol levels in child care rise from morning to afternoon (Tout, de Haan, Campbell, & Gunnar, 1998). This finding was later contrasted with the decreasing levels observed at home (Dettling, Gun- nar, & Donzella, 1999; Watamura et al., 2003). Disrupted patterns of cortisol secretion in child care could be of particular importance since early stress has been shown to shape the HPA reactivity over and above genetic factors in rodents (Weaver et al., 2004), in nonhuman primates (Barrett et al., 2009) and in human toddlers (Ouellet-Morin et al., 2008). By modifying their capacities to respond physiologi- cally to stress, individuals may experience greater difficulties in adapting to changing environments and be at higher risk for later psychopathologies. Cross-sectional studies suggest that atypical patterns of secretion in child care (rise or flat) emerge in late infancy, peak during the toddler years, and vanish during the early school years (Watamura et al., 2003). However, these studies mainly rely on heterogeneous samples of older children. Most importantly, given the cross-sectional Conflict of interest statement: No conflicts declared. Journal of Child Psychology and Psychiatry 51:3 (2010), pp 295–303 doi:10.1111/j.1469-7610.2009.02167.x Ó 2009 The Authors Journal compilation Ó 2009 Association for Child and Adolescent Mental Health. Published by Blackwell Publishing, 9600 Garsington Road, Oxford OX4 2DQ, UK and 350 Main Street, Malden, MA 02148, USA