NIH - Medical Encyclopedia Bipolar disorder

"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."

Clinical features of bipolar depression versus major depressive disorder in large multicenter trials. (Am J Psychiatry. 2006)

"RESULTS: Bipolar depression was associated with family history of bipolar disorder, an earlier age at onset, a greater previous number of depressive episodes … Fears were more common in patients with bipolar disorder, whereas sadness; insomnia; intellectual (cognitive), somatic (muscular), respiratory, genitourinary complaints; and depressed behavior were more common in patients with unipolar depression."