Bipolar disorder is characterized by periods of excitability (mania) alternating with periods of depression. The "mood swings" between mania and depression can be very abrupt.
Manic depression; Bipolar affective disorder
Bipolar disorder affects men and women equally and usually appears between the ages of 15 and 25. The exact cause is unknown, but it occurs more often in relatives of people with bipolar disorder.
Bipolar disorder results from disturbances in the areas of the brain that regulate mood. During manic periods, a person with bipolar disorder may be overly impulsive and energetic, with an exaggerated sense of self. The depressed phase brings overwhelming feelings of anxiety, low self-worth, and suicidal thoughts.
There are two primary types of bipolar disorder. People with bipolar disorder I have had at least one fully manic episode with periods of major depression. In the past, bipolar disorder I was called manic depression.
People with bipolar disorder II seldom experience full-fledged mania. Instead they experience periods of hypomania (elevated levels of energy and impulsiveness that are not as extreme as the symptoms of mania). These hypomanic periods alternate with episodes of major depression.
A mild form of bipolar disorder called cyclothymia involves periods of hypomania and mild depression, with less-severe mood swings. People with bipolar disorder II or cyclothymia may be misdiagnosed as having depression alone.