Oh. I know this one.
I know I'm tired when I have to keep reassuring myself that I am in reality, and that everything I'm doing is actually real. I'll start to half believe that I am asleep, or in a game, or part of some plot that I can change or stop doing at any time. And I have to keep reminding myself that I am awake, and this is real, and it will actually matter if I hurt myself.
This has happened a few times when I'm at work. It can be bad when I am using a knife that I've sharpened to have Super-Sonic-Samurai sharpness.