Well, in order to give a sincere apology, you might have to come out of the closet. When I get totally whacked out among people who know, I just say something like, "Sorry, i'm a bit on the manic side today" or 'My depression is running amok' or any one of a bunch of pre thought out sentences.

If you don't want to come out of the closet, then think up some sincere lines to use, so you have them ready if you need them.