I don't know if this make you feel any better or not, but I found this on the BP page on the National Institute of Mental Health, www.nimh.nih.gov :
Bipolar disorder causes dramatic mood swings from overly "high" and/or irritable to sad and hopeless, and then back again, often with periods of normal mood in between. Severe changes in energy and behavior go along with these changes in mood. The periods of highs and lows are called episodes of mania and depression