"Bipolar disorder is a serious brain illness. It is also called manic-depressive illness. People with bipolar disorder go through unusual mood changes. Sometimes they feel very happy and "up," and are much more active than usual. This is called mania. And sometimes people with bipolar disorder feel very sad and "down," and are much less active. This is called depression. Bipolar disorder can also cause changes in energy and behavior." (Quoted from article)