Asleep in Your Arms, Awake the Moment You Put Them Down
The short answer
Parents joke that their baby has a sensor on its back. But an infant's chest is not a small version of an adult's, and lying on the back genuinely makes the work of breathing harder. Waking up and not settling again appear to have different causes.
Contents
"My baby must have a sensor on their back."
Fast asleep in your arms, eyes open the moment you set them down. There is hardly a parent who has not said this. I think there is a reason on the body's side.
A baby's chest is not a small version of an adult's
In an adult the ribcage is fairly firm. Because that firmness holds, air comes in when the diaphragm descends.
In a baby under one year the chest wall is about three times more yielding than the lungs. It takes until around two years for the balance to become adult-like. The ribs also sit closer to horizontal rather than sloping down as an adult's do, so lifting them to widen the space does not work well.
So a baby leans heavily on the diaphragm. And in that diaphragm, the fibres that resist fatigue amount to about a tenth of an adult's. They have less to hold with, and more to hold.
Lying on the back costs more
There is a study that measured 20 healthy full-term newborns. Comparing lying on the back with lying prone, both the volume taken in with each breath and the volume per minute were lower on the back.
The study carries an index for how laboured the breathing is. On the back it read 2.0; prone, 1.3.
One thing here is worth noting. The measurement was made while the babies were awake and active — the very state a baby is in at the moment a parent puts them down.
This difference is not marked in every baby. Several studies found none. Position matters most to babies whose breathing reserve is already thin.
But the moment of waking is another story
There is an older answer for why they wake when put down: the Moro reflex — the response where a baby whose head tips back flings the arms out and draws them in again.
How this reflex is checked in the clinic is telling. The baby is held lying on its back and the head is let fall slowly — the act of laying a baby down is itself the standard stimulus.
The cost of breathing builds over minutes. The reflex is instant. So the eyes opening the moment you set them down is better explained by the reflex.
I read it in two layers
Waking and failing to settle again are different things.
The Moro reflex is over in seconds. What parents actually struggle with is not "she wakes when I put her down" but "once she's down she won't stay asleep."
What wakes them is the reflex; what keeps them from settling is the cost of breathing.
Light sleep takes up as much as two thirds of a baby's sleep, and in it the muscles between the ribs lose their tone. While awake those muscles hold the yielding chest wall; once that sleep comes the holding weakens. Lying on the back is less the cause of waking than a condition that lowers the threshold for it.
Why being held works
Being held is usually closer to prone or on the side. And something is added: the body in contact supports the baby's chest wall from outside.
A baby's chest wall is yielding enough that outside pressure affects it. A firm swaddle appears to help for the same reason.
Holding is a chest-wall support a person makes. What parents found by experience has grounds on the body's side.
Do put them on their back to sleep, though. The position that is easier for breathing and the position that is safe for sleeping are different questions. Sleeping prone is not advised. Give tummy time while they are awake, and use a swaddle instead.
What I ask
When a child finds lying down unusually hard, I do not put it down to habit or temperament alone. I look at whether conditions that genuinely make breathing harder have gathered. I ask whether the nose is blocked, whether there is noise in sleep, whether feeding leaves them short of breath.
The same words — "she won't lie down" — do not point to the same thing in every child.
Sources
- A study comparing breathing on the back and prone in 20 healthy full-term newborns — Pediatric Pulmonology, 1994
- A study measuring how the yielding of the chest wall changes around the first year — Journal of Applied Physiology, 1995
Written by Dr. Heo Ji-young (Ph.D. in Korean Medicine Pathology, Kyung Hee University · former Research Professor of Herbology, Kyung Hee University)
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