Researchers at the Great Western Hospitals NHS Foundation Trust placed babies in a simulator to recreate a car journey.
After half an hour sleeping in the seat, the amount of oxygen in the blood of babies aged under two months “dropped significantly”.
“Babies do not have the muscle tone to support their heads and sleeping in a car seat may push the baby’s head forward,” the scientists, led by Dr Peter Fleming, of Bristol University, warned, according to the Daily Mail.
The study, funded by the Lullaby Trust with support from Southampton and Bristol universities, studied 40 infants overall.
They tested the heart and lung functions of the babies for 30 minutes in the simulated car replica. The experiment was stopped immediately if any babies started to suffer ill effects and trained medical staff with resuscitation equipment were on hand.
The authors found “significant” drops in the babies’ oxygen levels, as well as noting that their heart rates increased.
Researchers noted that the babies who experienced high drops in oxygen had “flopped forward” intermittently.
The authors said their findings suggested death “could happen, although it would be a very rare event”.
They advised an adult should always sit next to a baby to make sure they are breathing properly.
Commenting on the study, a spokesperson at Good Egg Safety, a company which offers car safety advice for families, told The Huffington Post UK: “We advise that infants should not be in a car seat for longer than 90 minutes.
“For longer car journeys we advise planning in regular breaks and taking your baby out of the car seat for a little while.”
They said the best position for a newborn is to be laid flat, but rear-facing seats are fine if they’re “more upright”, for better protection in a collision.
“There are some things you can do to try prevent a child’s head dropping forward in the car,” they continued.
“Before buying, try a few seats in the car and select one that gives a good recline position for a newborn.
“When using the seat, make sure it is installed correctly as per the manufacturer’s instructions, put your baby in the seat with their nappy touching the crotch strap (remember no puffy pram suits or coats).
“The harness should be at the correct height and tight so that you cannot fit more than two fingers between the harness and their collar bone.
“Most infant carriers come with inserts for better newborn positioning and these should always be used.”
Further advice can be found on the Good Egg Safety website.