Cortisol regulation in 12-month-old human infants: associations with the infants' early history of breastfeeding and co-sleeping.

Experiences during early life are suggested to affect the physiological systems underlying stress responses, including the hypothalamic-pituitary-adrenal axis (HPA axis). While stressful early experiences have been associated with dysregulated HPA-axis functioning, positive early experiences, i.e. high maternal caregiving quality, contribute to more optimal HPA-axis functioning. Influences of other early caregiving factors, however, are less well documented. The goal of this study was to examine whether breastfeeding and co-sleeping during the first 6 months of life were associated with infant cortisol regulation, i.e. cortisol reactivity and recovery, to a stressor at 12 months of age. Participants were 193 infants and their mothers. Information on breastfeeding and co-sleeping was collected using weekly and daily sleep diaries, respectively, for the first 6 months of life. Co-sleeping was defined as sleeping in the parents' bed or sleeping in the parents' room. At 12 months of age, infants were subjected to a psychological stressor [Strange Situation Procedure (SSP); Ainsworth et al. 1978]. Salivary cortisol was measured prestressor and at 25, 40, and 60 min poststressor to measure reactivity and recovery. Regression analyses showed that after controlling for maternal sensitivity, infant attachment status, feeding, and sleeping arrangements at 12 months of age and other confounders, more weeks of co-sleeping predicted lower infant cortisol reactivity to the SSP. Also, more weeks of breastfeeding predicted quicker cortisol recovery. These results indicate that an early history of co-sleeping and breastfeeding contributes positively to cortisol regulation in 12-month-olds.