This is an article from Curious Kids, a series for children. The Conversation is asking kids to send in questions they’d like an expert to answer. All questions are welcome – serious, weird or wacky!

Why do we always fall asleep when we’re in cars or moving vehicles? – Christian, age 7, Brisbane.

That’s a great question, Christian. There are a few possible reasons.

One is that we might be trying to catch up on the sleep that we didn’t get the night before, which is called our sleep debt.

Our bodies know when we are tired and so they will find times and places to sleep whenever they can.

But then why doesn’t everyone fall asleep in a moving car?

That’s partly because not everyone needs the same amount of sleep. Sleep scientists say that some children in your age group need only nine hours of sleep at night, while others need as much as 11 hours. It depends on the person.

And not everyone gets the sleep they need. If you have slept as much as you needed to the night before, you will probably not fall asleep in the car.

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Also, some people can handle being tired and can make themselves stay up much more easily than other people, just because they are born that way.

We may also fall asleep in a moving car because it is very similar to our beds. It’s warm and comfortable and we are usually feeling safe and relaxed. So if you are sleepy, comfortable and feeling safe, you will likely fall asleep in the moving car.

Sitting in a moving car can be boring. Usually in the day time, we are busy doing things that interest us and keep us busy. But in a moving car, you are not doing anything else except relaxing! Just watching boring things go past, not thinking of anything in particular.

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 bed time. This is sometimes called highway hypnosis and can happen to drivers too.

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The gentle rocking movement of the car can make us sleepy. Sleep scientists say that rocking or slow, gentle movements can make us fall asleep if we are tired, just like when we are babies and our parents rock us to sleep. It might remind us of when we were in our mother’s tummy.

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. It is a type of uninteresting, constant noise that seems to help us fall asleep. Many parents use humming white noise, like fan noises, to help babies fall asleep. And some grown-ups use white noise to help themselves get to sleep, too.

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.

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Sarah Blunden does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.