Breastfeeding but not co-sleeping has the best affect on the welfare of a mother, according to a new study released at the American Public Health Association meeting this week.

The study from the School of Education and Social Policy at Northwestern University looked at a mother’s level of the stress hormone cortisol in several different scenarios: breastfeeding but refrained from sharing a bed with their baby; not breastfeeding and sharing a bed; and breastfeeding and sharing a bed. Mothers who breastfeed but were not co-sleeping had the biggest decline in cortisol levels.

Researchers used saliva samples from 195 new mothers in a Chicago suburb.