Also known as manic depression, this is a mood disorder in which the person's mood swings from euphoric, manic stages to depressed (from one "pole" to the other). This is not simply being happy and then sad, but rather periods of uncontrollable, clinical mania and longer periods of depression. Although there are many different perspectives about the basis of bipolar disorder, the most popular view is that it is caused by a chemical imbalance. During periods of mania, the person may become incoherent, become irrational, hyperactive, unrealistic about themselves and others, and act in sexually, socially, and physically unhealthy ways (sleeping with many people, going on shopping sprees which they can't possibly afford, etc.).