Character

I wonder why physical and mental exhaustion can make someone critical, sensitive, and generally mean. When I'm over-tired, I can sometimes tell when I'm starting fights, nit-picking, or being nasty. As strange as it is, I often have trouble just keeping my mouth shut. I'm aware of what I'm doing and how unproductive it is, yet it's like I can't snap out of it. After the damage is done, I can go back and apologize (if I have the courage to do so). Unfortunately, this doesn't happen till a nice evening has gone sour. When I'm not the tired one, it comes at me from left field when the tables are turned. I'm still not quite sure how to handle this in any kind of decent way.

Is this a character flaw, or is this true character being revealed? I can see it both ways. If it's just an inappropriate way of responding to the electrical impulses coursing through the brain, then it could be corrected with conscious effort. However, if this is a person's true nature, then what?