I saw a question on a quiz recently, that asked "Do you find it hard to remain polite in social situations, when you are tired?" and I thought "Well yeah, who doesn't?" Social situations require a certain level of patience, a level which is usually unaccessible when you are tired. I often end up saying rude things or acting inappropriately when I'm tired. When I reach a certain level of exhaustion, I just give up caring and lose all my patience. Sometimes talking for too long tires me out, and when that happens I like to leave before I end up saying something I'd regret later.