Sitting for 7 hours in an 8.5 hour span is tough. Plus there is not full leg room typically.
Sitting at your office job or watching TV is different. Riding in a car or a plane for the same time would also have a similar effect.
Add to this the mental part you speak of, which I agree with, and that is a recipe to feel completely drained.
One more thing, I'm going to an assembly today. LOL