There is nothing else in the world that exhausts you more than that bleary-eyed walk in the thin light of morning to sooth your wailing baby. Over, and over and over again.
You curse that lost soother and you blame those pesky new teeth as you shuffle to the kitchen in the hope that a warm bottle will help.
But now, scientists have come up with a rather disconcerting alternative to why those little ones are bawling in the night.
They don't want a sibling.
Harvard scientist David Haig (who specialises in evolution biology) says the reasons behind it are obvious - another baby in the house means having to share the love/attention/food mum and dad currently lavish on their little one. Haig believes that biology steps in to programme baby to do everything they can to thwart the meeting of sperm and egg. The only tools in their baby arsenal is those surprisingly strong lungs!
Obviously, having mum available to nurse them during the night keeps her away from all sorts of other things that might result in a brand new brother or sister. (You know what we mean!)
Frequent interruptions and that exhaustion that comes with having a baby do not exactly help your poor hormone-bashed libido. Naturally, babies are programmed with survival instincts so often they simply wake up because they are too hot, or hungry, or they could just want a cuddle from mum or dad.
However, Haig says baby's self-interest means they don't want to share a good thing - this is the opposite of the mother's biological goal:
“Mothers have evolved to maximise their numbers of surviving children, which is different from maximising the survival of each individual child."
Unsurprisingly, not everyone agrees. But if Haig is right, it means that babies who cry at night might be showing the ultimate sibling rivalry.
Know anyone who has a crier? I'm sure they would love to hear this theory!