Breastfeeding and infant sleep
In a new article published online today in the journal Evolution, Medicine, and Public Health, Professor David Haig argues that infants that wake frequently at night to breastfeed are delaying the resumption of the mother’s ovulation and therefore preventing the birth of a sibling with whom they would have to compete. It has already been documented that smaller gaps between the births of siblings are associated with increased mortality of infants and toddlers, especially in environments where resources are scarce and where infectious disease rates are high, and Professor Haig believes that the benefits of delay are such that the selective forces are strong enough to have engendered a significant evolutionary response.
Professor Haig says, “The duration of postpartum amenorrhea is a major determinant of interbirth internals (IBI) in natural fertility populations with more frequent and more intense nursing, especially at night, associated with prolonged infertility. Natural selection will have preserved suckling and sleeping behaviours of infants that suppress ovarian function in mothers because infants have benefited from delay of the next birth. Maximal night waking can be conjectured to overlap with the greatest benefits of contraceptive suckling.”
D. Haig. Troubled sleep: Night waking, breastfeeding and parent-offspring conflict. Evolution, Medicine, and Public Health, 2014; 2014 (1): 32 DOI: 10.1093/emph/eou005
Those sneaky lil fucks