Bipolar disorder affects ~2.8% of U.S. adults (~5.7M, NIMH), often starting in late teens/early 20s. It involves extreme mood swings: manic episodes (high energy, euphoria, impulsivity, little sleep) alternating with depressive lows (sadness, hopelessness). Types include Bipolar I (full mania), II (hypomania + depression), and cyclothymic.