"Tired" is an excuse that many
people use when they look like
crap. Some are legitimately tired, but the amount that are using it as an honest excuse are dwindling.
People also use Tiredness when they are annoyed but don't want to seem like total jackasses.
Friend 1: Dude, whatsup with you? You look like
crap.
Person: Can you
shut up?
Friend 1: What's wrong man?
Person: Sorry, i'm just tired.