Studies show that the more decisions you make
in a row, the worse your decisions get.
At a certain point in a writing day, you start writing crap. This may be why. Writing is all decision making.
The important thing is to recognize when you're about to start writing crap, and stop writing for a while.
The danger is, of course, that this becomes an excuse for procrastination. "I need to recharge the batteries," you say, while you play a few levels of Bastion, or blog, or tweet, or go see if Nate Silver has posted anything new about the election, or fold laundry. (And the laundry actually needs to get done!) Writing requires sitzfleisch
: the willingness to sit and hack it out even if it's not perfect, which it never is.
But still, at a certain point, it's time to call it a day. No one cares how many pages you cranked out in a day. (Unless you're in production!) They care how many good pages.
Now back to work, Alex. There might be one more good page in you...