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.

Maternal prenatal stress and cortisol reactivity to stressors in human infants.

Early life factors can shape the development of hypothalamic pituitary adrenal (HPA) axis. Maternal prenatal stress might constitute such an early environmental factor. As little is known about the relation between maternal prenatal stress and cortisol reactivity in human offspring, we performed a longitudinal study including four assessments of infant cortisol reactivity to stressful events in a non-clinical population. General and pregnancy-related feelings of stress and anxiety, as well as circadian cortisol levels, were measured in 173 mothers in the last trimester of pregnancy. Infant cortisol reactivity was measured at 5 weeks to a bathing session, at 8 weeks to a vaccination, at 5 months to a stressful mother-infant interaction (still face procedure), and at 12 months to a maternal separation (strange situation procedure). Maternal prenatal fear of bearing a handicapped child was a consistent predictor of infant cortisol reactivity. Although the effects were mild, higher fear was significantly related to higher salivary cortisol reactivity to the bathing session and to decreased cortisol reactivity to vaccination and maternal separation. Thus, pregnancy-specific anxieties predict infant cortisol reactivity in the first year of life, but the direction of the effect depends on infant age and/or the nature of the stressor. While this specific anxiety was a better predictor than stress experience or maternal cortisol concentrations, the underlying mechanisms of these associations remain unclear. Future studies should try to incorporate multiple measures of HPA-axis reactivity during development when studying the long-term consequences of maternal prenatal stress.