When I was pregnant I worried about what would happen if the baby cried for me while I was in a deep sleep. Like so many pregnancy worries, though, blocking out my baby's cries was something I didn't really need to be concerned about. An alarm clock can go off inches from my head and I'll sleep through it for hours, but if my baby cries at the other end of the house, I'm wide awake.
It turns out, the sound of my baby crying impacts my brain very differently than a beeping alarm.
I'm hardly the first parent to make this observation, and science is on to it, too. There's plenty of research about how a baby's cries impact its mother on a physical level. A study of mother mice published in Nature found that adding oxytocin (a hormone released in strong doses during labor and lactation) to the brains of the mamas changed the way they processed the sound of crying pups—and helped them learn how to recognize and respond to the sounds.
A dose of this “motherhood hormone," it seems, leads to increased sensitivity to the sound of your child in distress.
According to Robert Froemke, that study's senior investigator, this suggests oxytocin amplifies the way the auditory cortex processes incoming cries from our own babies. He says the same seems to be true for female mice as female humans: The sound of a crying baby stirs up a great sense of urgency.
This physiological reaction allow us to develop rapid, reliable behaviors to our babies' cries, says Froemke. In time, it also helps us learn what the cries mean—and how we can respond in a helpful way.
When our babies cry, “[as parents, we] don't know what's really going to work, we just try a bunch of stuff. Let's change a diaper, let's feed the baby, let's do a little dance," he says. “Eventually we learn this repertoire of parenting skills because we're all in, we're all invested and that baby depends on us absolutely to take care of it."
Researchers believe that it may be this hormonal shift in the brain that alerts a mother to the sound of her child's cry.
Mothers' brains have a different level of sensitivity to crying babies
In humans and in mice, dads often respond to a baby's cries, but the brain chemistry is a little different: According to Froemke, extra oxytocin doesn't speed up the reaction to crying pups in male mice the way it does for females.
"There is a difference in terms of [ a father's] sensitivity to oxytocin. We think that may be because the male oxytocin system is already maxed out," he explains, adding there is something about living with a female and child that contributes to a natural oxytocin increase in mouse dads. (Further proof moms aren't the only ones to deal with big hormone changes.)
But when it comes to the brains of human parents, there is more evidence that the brains of men and women respond to crying babies differently. A study published in NeuroReport looked at the brains of 18 men and women who heard a baby crying while inside a brain scanner. The women's brain activity suggested an immediate alertness, while the men's brain activity didn't change.
That study suggests there are gender differences in the way we process baby sounds, but a lot of dads will tell you they can't and don't sleep through a baby cries. And that's for good reason: According to Froemke, it's no biological accident that babies signal distress in a way that can pierce parents brains even when our eyes are closed.
"Parents have to sleep, too," he says, but, "Sounds penetrate our brains, they tap into something deep and we can quickly rouse from a deep slumber, jump out of bed and tend to infant needs."
Just as my son is biologically wired to be my personal alarm clock, I am biologically wired to hear him—even if I can still sleep through everything else.
[Originally published October 18. 2017]
