A Harvard evolutionary biologist thinks babies’ nighttime crying spells are a form of birth control.

David Haig hypothesizes that babies demand food at all hours of the night to exhaust their mothers and prevent them from ovulating. This works to the baby’s advantage, Haig believes, because if the mother can’t become pregnant and have another child, the baby is likely to get more attention and more food and improve its chances for survival.

In our modern-day society this seems a little far-fetched, but hundreds of years ago, when birth control didn’t exist and people hunted and gathered for their food, Haig’s evolutionary theory is plausible.

Today’s society more commonly assumes that babies wake in the night because they need food, but Haig claims that babies get plenty of nourishment even if they sleep through the night. Around-the-clock feedings are entirely unnecessary for babies to maintain a healthy weight. Haig theorizes babies cry to keep their mothers infertile. In a paper for Evolution, Medicine and Public Health, he writes “babies waking at night to suckle is an adaptation of infants to extend their mothers’ lactational amenorrhea.”

Lactational amenorrhea — the temporary infertility that occurs when a woman is not menstruating and fully breastfeeding — is a controversial topic. Many women assume they’re not ovulating in the first few months after giving birth so they don’t go on birth control or use protection—and then they end up getting pregnant. Nursing isn’t a fool-proof form of birth control. But according to the Mayo Clinic, within the first 6 months postpartum, lactational amenorrhea in women who are exclusively breastfeeding is 98 percent effective in preventing pregnancy, according to the Mayo Clinic.

The scientific community isn’t entirely buying into Haig’s theory. University of Rhode Island anthropologist Holly Dunsworth pointed out in an NPR segment that babies feed at night for many reasons. For example, Dunsworth says, nighttime nursing offers the opportunity for mother and child to bond.

“There are so many good juices running through infant and mom,” Dunsworth told NPR. “It’s rewarding beyond the calories and hunger satiation for everyone involved. … When you look at it from that perspective, waking up to feed looks more like cooperation than conflict.”

Harvard evolutionary biologist Katherine Hinde brought up another important point.

…babies evolved to cry at night thousands and thousands of years ago when people’s sleeping habits weren’t like they are today. Back then, people weren’t working 9 to 5 in offices and trying to get eight hours of sleep. Instead, we were likely hunters and gatherers whose sleep schedules were flexible and likely fragmented. “The expectation that mothers and infants ‘should’ have uninterrupted, consolidated sleep is, in many ways, a historical artifact,” Hinde in the same journal, in a response to Haig’s idea.

One more thing to consider: Babies nighttime crying jags might also be working as a form of birth control by not allowing mom and dad to get intimate. Fitting in baby-making time in between newborn feedings is next to impossible.