Bipolar disorder is a mental illness that can cause significant shifts in mood and energy, including depressive lows and manic highs. Bipolar symptoms usually become apparent during late adolescence or early adulthood. Symptoms can be triggered by events, as well, such as pregnancy and childbirth. 

Depressive episodes are signified by feeling "down," sad, indifferent, or hopeless. Signs of a manic episode include feeling very energized, elated and/or irritable. Manic or depressive episodes may happen fairly rarely, up to multiple times a year.

Individuals with a diagnosis of Bipolar Disorder I tend to experience episodes of mania as well as depressive episodes. A diagnosis of Bipolar Disorder II is characterized by episodes of depression and episodes of hypomania (an elevated state that is less intense and lengthy than a manic episode).