We actually talked about this at a meeting at work. Like some others have said, part of the fatigue is from all these little muscles in your body that help you balance have to work constantly.
Being that I drive for 8 hours a day, 5 days a week for a living, I know very well this tired feeling that is described. Stretches, and physical activity definitely help. When I have breaks, if I’m not hungry/need to go to the bathroom, I tend to walk around for as long as possible. Sometimes when I’m tired I lie down in the back seats of the bus just to give those muscles a break for a few minutes. It helps.