When I get tired, my mind shuts out coherent thoughts and I cease to hear. My eyes blur everything out of focus. Even if I'm staring at the TV, I see and hear nothing. When I'm tired, I stop talking. My blinks become slower and longer (and between you, me and the internet, I get cranky).
When Jim gets tired, he scratches his head.
When he's not tired, he's ornery and does things like this.
Do you see it?