Bipolar disorder is a mental illness that causes extreme mood swings. These mood swings can result in periods of mania, during which a person may feel overly happy and energetic, and periods of depression, during which a person may feel very sad and hopeless. Bipolar disorder can also lead to changes in thinking and behavior, as well as problems with relationships and work. The cause of bipolar disorder is not known, but it is thought to be related to changes in the brain. Bipolar disorder appears to be genetic, meaning that it runs in families. Bipolar disorder can also be triggered by stressful life events.