As you know, many writers suffer from writers block. And if you're unfamiliar, it's simply a case where the writer is unable to proceed in their work due to a 'block' in their ability to create.
I do not suffer from this case, but I have found something similar.
This will happen usually after the 2000-2500 word mark, almost every day I write. My eyes will grow lazy, my vision will blur, and I will have the urge to play 'Demise: Ascension'.
I believe it's because my mind has acted like a conduct for too long, much like Joe Konrath describes in his blog: http://jakonrath.blogspot.com/.
My fellow writers in my writing group have yet to suffer from this particular type of block, but I think it is all too common when you immerse yourself in a world where you brain has to do all of the heavy lifting. The equivalent of a strenuous workout for your noggin.
I mean, making worlds isn't easy, and when you write with what I call the 'conduct' method, you're essentially letting your imagination run wild, and you cherry-pick the results.
Writing is fun, and if you read Konrath's blog, he will consistently say "WRITE MORE". So that's what I'm trying to do, but man, it's tough to keep 'tuned in' for extended periods...