It is more than just a simple mood swing.
Mixed states can also occur.
It is estimated that one in 100 people suffer from manic-depression. It generally strikes before 35 years of age. The length and frequency of cycles between the low of depression and the high of mania vary from person to person, as do the other symptoms of manic-depression. Recent studies have led scientists to believe genetic changes in body chemistry may be a cause of manic-depression. Researchers believe bipolar patients' neurotransmitters - chemicals that allow brain cells to communicate, are affected during phases of the disease.