Bipolar II disorder is a form of mental illness that involves periods of depression and periods of elevated mood, called hypomania. It is similar to bipolar I disorder, with moods cycling between high and low over time, but the "up" moods never reach full-blown mania. A person affected by bipolar II disorder has had at least one hypomanic episode in their life, and most people with bipolar II disorder have episodes of depression more often. The less-intense elevated moods in bipolar II disorder are called hypomanic episodes, or hypomania. During a hypomanic episode, a person may feel elated, energized, or even irritable, and the symptoms usually last for a few days. The causes of bipolar disorder are not well understood, but research has shown that it can have a genetic component and run in families. Multiple factors, including stress, lack of sleep, and alcohol or substance abuse, may interact to trigger abnormal brain circuit function. Bipolar II disorder is not a milder form of bipolar I disorder, but a separate diagnosis. While the manic episodes of bipolar I disorder can be severe and dangerous, individuals with bipolar II disorder can be depressed for longer periods, which can cause significant impairment.