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The cause of bipolar disorder is not well understood. However, it appears that brain chemistry plays a major role in its development. Brain chemicals called neurotransmitters convey messages between the nerves.  Too many or too few neurotransmitters are believed to cause alterations in mood.

Bipolar disorder appears to be linked to genetics, and great strides are being made in identifying genes associated with the condition. Individuals who have relatives with a history of a mood disorder are at a higher risk of developing bipolar disorder. This is especially true for first degree relatives (e.g., parents, children, siblings). However, this is not a definite way of determining who may eventually develop bipolar disorder. For example, if one identical twin has the disorder, the other has an increased risk of developing it but may never actually do so. Some researches believe that some people inherit a tendency to develop the illness, which may then be triggered by environmental factors (e.g., stressful life events, disturbances in circadian or seasonal rhythms). Other possible triggers include sleep deprivation and treatment with antidepressants and certain herbal and dietary supplements (e.g., St. John’s wort).

Depression appears to be closely associated with bipolar disorder. According to the American Psychiatric Association, about 5 to 10 percent of patients who have had a major depressive episode will eventually have a manic episode and be diagnosed with bipolar disorder.