Babies cry at night to prevent their mom from procreating and giving birth to a sibling, a Harvard University study speculates.
The hypothesis, published in the journal Evolution, Medicine and Public Health, ventures that babies who cry in the wee hours of the morning are just applying a survival of the fittest technique - whether consciously or unconsciously.
The authors say that by keeping their parents tired, and therefore delaying conception, they are more likely to survive.
"It's clear that babies can get enough milk even if they sleep through the night," evolutionary biologist David Haig told Shots. "The waking becomes a different issue. ... I'm just suggesting that offspring have evolved to use waking up mothers and suckling more intensely to delay the birth of another sibling."
What society deems as typical baby behavior - crying when you're supposedly hungry - is actually an innate evolutionary response, the researchers say.
During times of food shortage and disease, a baby has a better chance of surviving if there are no other siblings for the mother to feed and care for. Delaying a mom's fertility post-pregnancy - known as amenorrhoea - by crying at night may have helped ensure survival.
But anthropologist Holly Dunsworth says that this ritual may not just be beneficial to the baby, but to the mom as well, encouraging love and affection.
"There are so many good juices running through infant and mom," Dunsworth adds. "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."
- I never planned to be a stay-at-home dad.
- Babies Can Fake Crying To Gain Attention, Helps Them Get Closer To Parents
- Chinese Baby Pronounced Dead Starts Crying Just Before Cremation
Furthermore, humans' sleeping habits were much different historically than they are today.
"The expectation that mothers and infants 'should' have uninterrupted, consolidated sleep is, in many ways, a historical artifact," evolutionary biologist Katherine Hinde wrote.
But this secret agenda babies may have is just one theory - and one that cannot be tested.
"It's an interesting perspective," Dunsworth concluded, "but it's not the only one."