Your body is actually doing a fair amount of work in a vehicle like a car, even when you’re “resting”, to compensate for the movement you’re experiencing and, especially, any subtle changes in it. Add to that that often you are also exerting the brain, especially on longer trips, and it can add up to a decent expenditure of energy that can make you pretty tired, especially if you are adding some amount of monotony fatigue to it.
Latest Answers