When I'm tired, I make mistakes. It's probably a bigger mistake to let myself get so tired in the first place, because it leads to things like repeatedly tapping the wrong key while entering spreadsheet data. I've known how to type since seventh grade, but when I'm tired sometimes I can't find Home Row on the keyboard, and asdfjkl; becomes xcvbuiop or tqwe7890. From there, things can only get worse.
When I'm tired, small mistakes and petty problems become great dramatic tragedies. I can break a pencil lead or drop a piece of paper, and a pathetic display of hand-wringing and teeth-gnashing will break out. (Actually, I don't wring my hands or gnash my teeth, not literally. But I do whine and cry a lot. When I'm tired.)
I also tend to bang my knee on the corner of the desk more often when I'm tired.
When I'm tired, I don't make the best decisions. Instead of working all day on the report I need to have done by Monday morning, I spent the morning on something else. It was something equally important in the long run but much less time-sensitive in the short run.
Why did I do this? I don't know, but I was too tired to think about the consequences (which are that I now have to work after I get home from San Francisco tomorrow, because I can't, just can't, work any later tonight, especially since I have to get up early Sunday morning to catch the bus for the City.)