Bipolar disorder, also known as manic-depression, is a chronically recurring condition involving moods that swing between the highs of mania and the lows of depression. But that's not quite the whole picture; depression is by far the most pervasive feature of the illness, while the manic phase usually involves a mix of irritability, anger, and depression, with or without euphoria. The elation may manifest as unusual energy and overconfidence, playing out in bouts of overspending or promiscuity, among many other behaviors.
The disorder most often starts in young adulthood, but can also occur in children and adolescents. Misdiagnosis is common; the condition is typically confused with everything from attention-deficit/hyperactivity disorder to schizophrenia to borderline personality disorder. Biological factors probably create vulnerability to the disorder, and experiences such as sleep deprivation can kick off manic episodes.