A baby who wakes repeatedly at night seems interested only in milk and comfort. But one provocative evolutionary hypothesis suggests that night waking may also delay the arrival of a younger sibling.
Frequent breastfeeding suppresses the hormonal cycle that leads to ovulation—a phenomenon called lactational amenorrhea. Nighttime nursing may be especially influential. By waking and suckling, an infant could extend the interval before its mother conceives again, postponing competition for milk, attention and protection.
This is an example of parent–offspring conflict. Mother and child share many evolutionary interests, but not perfectly. A mother’s reproductive success may favor eventually investing in another child; the current infant benefits from receiving undivided care for longer.
Strongest supporting evidence: breastfed infants generally wake more frequently during the second half of their first year, while frequent nursing helps prolong infertility. Certain rare genetic disorders involving genomic imprinting also produce opposite sleep patterns depending on whether the affected genes are active copies inherited from the mother or father—indirectly consistent with competing parental genetic interests. Evolutionary hypothesis
Well supported: infant waking, nighttime feeding and lactational suppression of ovulation are real biological phenomena.
Plausible but highly debated: babies evolved to wake specifically to delay a sibling. No study has demonstrated that night waking actually increased ancestral infant survival through longer birth intervals. Babies also wake from hunger, immature sleep rhythms, illness, discomfort and a need for contact. Comparative evidence shows that intensive nursing does not always prevent rapid maternal reproduction. Critical response
The hypothesis is fascinating—not because babies consciously scheme, but because natural selection can create conflicts without anyone intending them.
