Everyone experiences the "blues" now and then as well as times of joy and self-confidence. Most people even experience mood swings--times when they move quickly from feelings like joy to opposite feelings like sorrow. But what happens when normal moods become so extreme that a person can't think, feel, or act appropriately? What if a person is so "up" he does foolish, even dangerous, things? What if he's so "down" he can barely get out of bed?