This could be because sitting in a moving car can be very boring.
“Usually in the daytime, we are busy doing things that interest us and keep us busy," writes Sarah Blunden, Associate Professor and Head of Paediatric Sleep Research, CQUniversity Australia. "But in a moving car, you are not doing anything else except relaxing."
She added: “This is the same as sleep time in your bed. Our minds and bodies are not doing anything except getting ready for sleep, so they become quiet and calm. So in a moving car, your mind and body can go into the same kind of quiet “daze” as they do at bedtime. This is sometimes called highway hypnosis and can happen to drivers too.”
Another cause of children falling asleep in cars could be to do with the gentle rocking of the car moving.
“When we are in a moving car, there is a gentle and constant humming noise from the car engine. Sleep scientists call this white noise,” Blunden said.
“Sleep scientists don’t know exactly why young babies seem to calm down and fall asleep when hearing white noise, but the hum of the car engine in a moving car is a type of white noise.”