A Harvard study titled, "Troubled Sleep," led by David Haig found that babies cry at night to stop parents from having sex or create another baby. The study was published in the Evolution, Medicine and Public Health Oxford Journals.

The study said crying and night walking are a mechanism for babies to at least postpone their parents from producing siblings. Haig said those breastfed babies who cry frequently for nighttime feedings want to extend their mothers' lactational amenorrhea or the women's bodies ability to stop conceiving as long as breastfeeding.

"Blurton Jones and da Costa proposed that night waking to suckle is an adaptation of infants to suppress ovarian function in their mothers, thereby delaying the conception of a younger sib with whom an infant must compete for parental care and attention."

Haig pointed out this is done by babies unconsciously. "No implication was intended that contraception was a conscious motivation of infants, but simply that infants who woke their mothers left more descendants. Neither was the resumption of ovulation implied to be a conscious maternal strategy to trade a decrement in probability of survival for an extra child, simply that more total offspring survived if IBIs were shorter than were best for the survival of individual infants."

The study boiled down to the comparison of breastfed and formula-fed babies and breastfeeding and bottle feeding during the night.

"If the function of night waking is to prolong lactational amenorrhea, and uninterrupted sleep has countervailing benefits, then waking would be maladaptive for infants whose mothers do not respond by nursing. The earlier onset of sleeping through the night in the absence of breastfeeding could thus be interpreted as the facultative quiescence of an ineffective function. Such an interpretation would imply that modern infants distinguish bottle-feeding from suckling. However, breastfed infants who are not nursed at night sleep longer than breastfed infants who are nursed at night even though both consume human milk."

In this case, Haig said breastfeeding contributed to trouble sleeping among babies.

"Waking, it would seem, is reinforced by breastfeeding and extinguished by the cessation of night suckling."

The researcher noted parental decision on child's well-being is unlikely to be irrevocably compromised by minor variations in parental care, whether parents choose to breastfeed or bottle feed.