Arrrggghh! Yesterday I was so tired I ended up trying to write and failing. Not because there wasn't anything to write -- the words were there. I just couldn't get my language center enough juice to power the sentence constructor. So instead I sat there stupidly looking at the screen and thinking nothing. So I wasn't sleeping AND I wasn't writing. Brilliant [/sarcasm].
I know the words were there because after I went out into the freezing cold to do something, I woke up and wrote for a few minutes until the tiredness crept back in.
I need a different strategy on weekdays. I just don't know what it is yet.