Babies have a tendency to cry at night, much to the dismay and frustration of many a parent. There are many reasons for why this might be: the baby is hungry, cold, wet, just wants to be held, someone made a noise that woke them up. However, a recent study described in a Science News article provides another reason for why this nighttime crying occurs: it prevents the baby’s mother from getting pregnant again.

Evolutionary biologist David Haig proposes in the past, babies that cried at night had a better chance of survival. This is because the constant interruptions have multiple effects on the mother, many of which act as a kind of birth control. In addition to interrupting potential late-night liaisons and making the mother exhausted, regular nighttime breast-feeding causes hormonal changes in the mother that can delay ovulation (though this is not a foolproof contraception method). Combined, these results of nighttime crying reduce the chances of the mother getting pregnant. How does this relate to the baby’s survival? The answer is simple: if the mother is not devoting resources to another child then the baby will receive more attention and care from the mother during a very vulnerable stage in life. Thus the baby will be more likely to survive its early childhood, and  therefore be more likely to reach reproductive years during which it can pass on its genes to offspring.

While many point out that babies crying at night is caused by a number of other factors, Haig’s proposal provides a new evolutionary significance for the phenomenon. Not only can nighttime crying be seen as sibling rivalry, as the baby is trying to avoid having more siblings for as long as possible, but also shows an example of the battle between sex-linked genes. Male genes tend to cause behaviors that will result in the passing on of genes to the next generation, regardless of the cost to the mother- which explains why male babies have a tendency to cry more at night than females, because there is no guarantee that the mother’s next baby will carry the male’s genes. However, females carry certain genes that tend to make them sleep longer at night as babies, which acts more in the interests of the mother who wants to have more energy to care for her young. Additionally, Haig points out that nighttime feeding is not actually necessary for the baby in our modern day society where most babies have access to adequate nutrients, which supports the idea of nighttime crying being a clever, though unintentional, survival strategy of babies – and also means that moms don’t need to feel too bad if they skip a nighttime feeding session or two.