It’s a routine new mothers know well: put the baby to sleep, hope the quiet will last, and then wake in the middle of the night to a chorus of wails and howls.

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It’s not obvious why babies disrupt the peace so frequently, particularly at night. Maybe they’re hungry, hot, or just plain cranky—but maybe it’s more complex than all that.

Evolutionary biologist David Haig has a new idea about why babies cry at night. He theorizes that they cry in order to distract their mothers and prevent future pregnancies. Frequent nighttime nursing delays a woman’s fertility, too, and can precipitate hormone changes that curb ovulation.

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Not all scientists are convinced that Haig is on the right track. Some argue that babies rouse in the night for more immediate needs like food, water, or comfort. Self-initiated crying could also be a survival technique that prevents a baby from sleeping too soundly.

Here’s Laura Sanders, writing for Science News: