I've said yes. Besides the physical fatigue (like my arm getting tired, or hands tired of holding the pencil) after a long stint, I can start getting too tired (or possibly more lazy) to bother with details and everything becomes a blobby mess.
My absolute longest time drawing is six hours straight, but that's a rare thing indeed.
For me, I wouldn't say tired. More like, concentrating on drawing is...uh...I lose that concentration very quickly. I'll draw and check Kotaku. Draw and check DA, draw, check whatever else, draw, stare out the window at the sky like I'm brain damaged. Not quite the same there but it's very annoying.
I ask because I've been drawing since 8am after a good 9 hour rest, it's 1:43 pm and I feel like I need another 9 hour rest. This happens to me alot. I find that the harder I work to get all the forms and what not right, I get tired. Like my mind is doing an uber workout or something.