Solitary sleeping in young infants is associated with heightened cortisol reactivity to a bathing session but not to a vaccination

Background: In this prospective longitudinal study, we investigated the relation between sleeping arrangements and infant cortisol reactivity to stressors in the first two post-natal months. Co-sleeping, as compared to solitary sleeping, is hypothesized to provide more parental external stress regulation by night, thus reducing general stress sensitivity. We therefore expected lower cortisol reactivity to stress in infants who co-slept more regularly. Methods: Participants were 163 mothers and infants from uncomplicated, singleton pregnancies. Mothers completed daily diaries on sleeping arrangements in the first 7 weeks of life. Co-sleeping was defined as sleeping in the parents' bedroom (i.e. own or parents' bed). Cortisol reactivity was measured twice: to a mild physical stressor (bathing session) at 5 weeks of age and to a mild pain stressor (vaccination) at 2 months of age. Results: Infants with a solitary sleeping arrangement in their first month of life showed a heightened cortisol response to the bathing session at 5 weeks compared to infants that co-slept regularly. This effect was not explained by breastfeeding practices, maternal caregiving behavior, or infants' night waking and sleep duration. No effects were found of co-sleeping on the cortisol response to the vaccination at 2 months. Conclusions: The results suggest that solitary sleeping in the first month of life is associated with heightened sensitivity of the HPA-axis to a mild stressor, possibly due to less nocturnal parental availability as external stress regulator. Whether this effect continues in later life, remains to be investigated. (C) 2011 Elsevier Ltd. All rights reserved.