Bipolar disorder, formerly known as manic depression or manic-depressive illness, involves alternating periods of mania or hypomania and periods of major depression, explains The Merck Manual Home Edition. Bipolar disorder is common, affecting about 4 percent of the population. Symptoms usually begin in adolescence or early adulthood.
Extreme mood swings different from a person's normal emotional state are the hallmark of bipolar disorder, according to the National Institute of Mental Health. Manic or hypomanic states include feelings of euphoria, extreme irritability, racing thoughts, restlessness and distractibility. The person may engage in high-risk behavior such as gambling, spending money excessively or having indiscriminate unprotected sex. He often needs very little sleep and may have unrealistic beliefs about himself or his abilities.
By contrast, a person in a depressive mood state feels sad, guilty and hopeless; experiences a loss of pleasure and interest in things he once enjoyed; and may have changes in his sleep, eating or other longstanding habits, states the National Institute of Mental Health. He may also have suicidal thoughts. Acting on suicidal thoughts is a serious risk associated with depressive episodes.
In both cases, these mood states last an extended period of time and cause disruption in the person's life, notes Mayo Clinic. The person may cycle rapidly between mood states as often as several times a week or as infrequently as several times a year.