Bipolar disorder, previously known as manic-depressive illness, is a brain disorder that causes unusual shifts in mood, energy, activity levels, and the ability to carry out day-to-day tasks.
Symptoms vary between people, and according to mood. Some people have clear mood swings, with symptoms of mania and then of depression each lasting for several months, or with months of stability between them. Some spend months or years in a "high" or "low" mood.
A "mixed state" is when a manic and a depressive episode happen at the same time. The person may feel negative, as with depression, but they may also feel "wired" and restless.