It is commonly described as a mental illness in medical journals
Bipolar disorder is a serious mental illness in which common emotions become intensely and often unpredictably magnified. Individuals with bipolar disorder can quickly swing from extremes of happiness, energy and clarity to sadness, fatigue and confusion. These shifts can be so devastating that individuals may choose suicide.
All people with bipolar disorder have manic episodes — abnormally elevated or irritable moods that last at least a week and impair functioning. But not all become depressed.
http://www.apa.org/topics/bipolar/