At the lower levels of mania known as hypomania, individuals appear energetic and excitable and may in fact be highly productive. At a higher level, individuals begin to behave erratically and impulsively, often making poor decisions due to unrealistic ideas about the future, and may have great difficulty with sleep. At the highest level, individuals can experience very distorted beliefs about the world known as psychosis. Individuals who experience manic episodes also commonly experience depressive episodes; some experience a mixed state in which features of both mania and depression are present at the same time. Manic and depressive episodes typically last from a few days to several months and can be interspersed by periods of "normal" mood.

Current research suggests that about 4% of people experience some of the characteristic symptoms at some point in their life. Prevalence is similar in men and women and, broadly, across different cultures and ethnic groups. Genetic factors contribute substantially to the likelihood of developing bipolar disorder, and environmental factors are also implicated. Bipolar disorder is often treated with mood stabilizing medications and psychotherapy. In serious cases, in which there is a risk of harm to oneself or others,involuntary commitment may be used. These cases generally involve severe manic episodes with dangerous behavior or depressive episodes with suicidal ideation. There are widespread problems withsocial stigma, stereotypes, and prejudice against individuals with a diagnosis of bipolar disorder. People with bipolar disorder exhibiting psychotic symptoms can sometimes be misdiagnosed as havingschizophrenia.

The current term bipolar disorder is of fairly recent origin and refers to the cycling between high and low episodes (poles). The term "manic–depressive illness" or psychosis was coined by German psychiatristEmil Kraepelin in the late nineteenth century, originally referring to all kinds of mood disorder. German psychiatrist Karl Leonhard split the classification in 1957, employing the terms unipolar disorder (major depressive disorder) and bipolar disorder.