"Thinking" is only one thing that the brain does, and it is actually a very small part of its job. All brain activity causes the brain to get tired and burn out to the point that it needs rest. Try driving a car around a racetrack competatively, even in a simulator. Talk to a ballet dancer about keeping their balance for hours. Talk to a soldier under fire hour after hour. Talk to a pilot trying to negotiate with ATC all during a weather event. All brain activity, even the basic biological stuff we don't classify as "thinking", causes the brain to get tired and eventually lapse into a survival/maintenance mode to handle the stress.
The driving one reminds me of how absolutely brain dead I feel after riding a motorcycle for 8-12 hours in a day. Nevermind that my body is rocked from the wind all day but just being able to do basic math or have a conversation with the person at the front desk of the motel I’m staying at becomes difficult. Any sort of sustained concentration obliterates your brain over a long enough time period.
Even driving a car can have similar effects after driving for 8-12 hours sans the wind and elements exhausting the body or the uncomfortable motorcycle posture
The wind and the posture aren't as much of a problem as the mental strain of riding a motorcycle. It is simply more difficult than driving, the constant gear shifting, cancelling turn lights manually and always being in a kind of defensive posture where you assume that you are invisible and someone might run into you.
I've always gotten into a kind of flow state while riding a bike. The exhaustion after hours of riding felt very similar to studying or coding all day.
Would the amount of brain power to do activity X decrease as you get experienced at it though? Not to zero of course (the long drive is tiring even for an experienced driver).
