Writing exhaustion

Is it just me, or is it hard to keep writing all day long?

I'm a little behind in my NaNoWriMo word count. I've been trying to write 2,000 words a day, and I ended up skipping a day to catch up on sleep, so today's assignment was to write 4,000. Well, I've gotten 2,500 done so far, but that still leaves a normal-amount 1,500 to finish up tonight.

And I'm feeling spent.

I took a year off a few years ago just to work on my first novel. I wrote every day, but I could never manage to write all day every day. Does anyone do that? Can anyone write for eight hours straight?

Maybe in the throes of a passionate Muse, but on a daily basis, I find it fatiguing to write even four hours in a row, at least in one medium and on one topic. Even switching to, say, a blog post, affords me and my poor brain a little break.

Just wondering if I'm alone in that.

I'll take a rest now and come back to finish up my words tonight. Carry on, all you NaNoWriMos and other fellow novelists!