I've wondered this often myself. I have a brief depressive episode often at 4pm and midnight. If I'm sad then, then I know it's related to that. I do think that bi polar can make typical moods worse. I often try to check my thinking. If I'm sad and realistic, it's a mood. If I'm irratated or depressed and am doing all or nothing or black and white thinking or displaying grandiosity, then I tell myself that it's probably my disorder. I hope this helps.