You wouldn't think you'd get all that tired from "just sitting and typing" (or standing and typing, as the case may be). What wears you out is all the thinking. While the brain accounts for only around 2% of your total body weight, it consumes 20% of the body's energy output, 20% of the body's oxygen intake -- that's on average -- and 15% of the heart's blood output. And the brain's cut of everything comes off the top; the body gets whatever's left. When you're hard at work writing, the brain's energy and oxygen consumption go up. By the end of the day, those little grey cells are pooped. All they want to do is lie on the couch and vege, and they've hogged so much energy that by then your body agrees with them!