Maybe not... but that's precisely what I did this morning. 

I was bound and determined to finish the chapter I'm currently working on. I set a Word Count Goal and by jingo I achieved it... despite the fact that I was pretty much sliding down in my seat from exhaustion and barely able to see the screen by the time I was done.

Was it smart to push myself like that? Probably not. Was it even good writing? Very definitely not! But to my mind it makes sense to get words on the page--because you KNOW you're going back there later on to fix all the errors and not-so-great writing.

Even if you don't have a chronic illness, a "meh" writing day gives you something to work with when you're better equipped. My advice is WRITE when you can, even if all that comes out is... let's just say something you're not proud of. I think you'll find that revision is when the "real" writing happens!