Bipolar disorder is a serious mental illness. People who have it experience dramatic mood swings. They may go from overly energetic, "high" and/or irritable, to sad and hopeless, and then back again. They often have normal moods in between. The up feeling is called mania. The down feeling is depression.
Bipolar disorder can run in families. It usually starts in late adolescence or early adulthood. If you think you may have it, tell your health care provider. A medical checkup can rule out other illnesses that might cause your mood changes.
Untreated, bipolar disorder can result in damaged relationships, poor job or school performance, and even suicide. However, there are effective treatments: medicines and "talk therapy". A combination usually works best.
NIH: National Institute of Mental Health