system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is driving (or passengering) so tiring?

My kids spent most of the day on a long car ride and came home exhausted. Which I get, because I'm the same way. Driving is more tiring, but even riding for such a long time takes a real physical toll.

Why? I'm just sitting there after all, and if I'm not driving I don't even have to pay attention. Why do I feel physically drained after a long drive?
gpt
Even as a passenger,  you're not really just sitting like you would at your desk. 

In a car your body constantly has to make micro adjustments to keep you upright, if you let yourself go completely, you move around your seat a lot even with a seatbelt. 

So all those micro adjustments that happen in a lot of places in your body (back, neck, etc) wear you down slowly and you don't really notice it until it's 2 hours in and you are exhausted.