Bipolar disorder, also known as bipolar affective disorder, manic-depressive illness, or affective psychosis, is a mental illness characterized by periods of elevated mood and periods of depression. The elevated mood is significant and is known as mania or hypomania depending on the severity. During mania an individual feels or acts abnormally happy, energetic, or irritable. They often make poorly thought out decisions with little regard to the consequences. The need for sleep is usually reduced. During periods of depression there may be crying, poor eye contact with others, and a negative outlook on life. The risk of suicide among those with the disorder is high at greater than 6% over 20 years, while self harm occurs in 30-40%. Other mental health issues such as anxiety disorder and drug misuse are commonly associated. The cause is not clearly understood, but both genetic and environmental factors play a role. Typically many genes are involved. Environmental factors include long term stress and a history of childhood abuse. It is divided into bipolar I disorder if there is at least one manic episode and bipolar II disorder if there are at least one hypomanic episode and one major depressive episode. In those with less severe symptoms of a prolonged duration the condition cyclothymic disorder may be present. If due to drugs or medical problems it is classified separately. Other conditions that may present in a similar manner include: drug misuse, personality disorders, attention deficit hyperactivity disorder, and schizophrenia as well as a number of medical conditions.
Evolutionary biologists have hypothesized that bipolar disorder could have come from an adaptation to extreme climactic conditions in the northern temperate zone during the Pleistocene. The Evolutionary Origin of Bipolar Disorder (EOBD) hypothesis states that during the short summers of extreme climactic zones, hypomania would be adaptive, allowing the completion of many tasks necessary for survival within a short period of time. During long winters the lethargy, hypersomnia, lack of interest in social activities, and overeating of depression would be adaptive to group cohesion and survival. Evidence for the EOBD hypothesis include an association between bipolar disorder and a cold-adapted build, correlation between seasonality and mood changes in those with bipolar disorder, and low rates of bipolar disorder in African Americans. The EOBD hypothesis suggests that in the absence of the extreme climactic conditions that fostered the success of bipolar disorder genes, many bipolar disorder behaviors are maladaptive and can often severely impair normal functioning.