The human body is an amazing thing. You think you can't be more tired, but then after a while you get used to that level and it seems like normal to you. Then the kids get sick and you move to the next level and wait for that to feel normal. I'm guessing that after a while you cease to be human and turn into some sort of drooling, crusty eyed freak whose hands are shaking so bad they won't hold a cup of coffee without it sploshing all over the place.
This can have one of two possible effects on my writing. 1) I can end up staring at the monitor while the drool dribbles onto the keyboard, or 2) my internal editor shuts down and I can write tons without that nasty voice saying "you suck." The second scenario is great when I have to write love scenes because there isn't anyone in my head saying "oh, don't write that, what if mum read it?"
Then again, it could be that what I write in that state is a pile of crap and I'm just too tired to know it. At this point, I think I'll just crawl back into bed and pray for fifteen minutes of sleep.