Disrupted sleep is probably the most common complaint of parents with a new baby. Night waking increases in the second half of the first year of infant life and is more pronounced for breastfed infants. Sleep-related phenotypes of infants with Prader-Willi and Angelman syndromes suggest that imprinted genes of paternal origin promote greater wakefulness whereas imprinted genes of maternal origin favor more consolidated sleep. All these observations are consistent with a hypothesis that waking at night to suckle is an adaptation of infants to extend their mothers' lactational amenorrhea, thus delaying the birth of a younger sib and enhancing infant survival.
Haig, DavidengEngland2014/03/13 06:00Evol Med Public Health. 2014 Jan;2014(1):32-9. doi: 10.1093/emph/eou005. Epub 2014 Mar 7.