Bipolar disorder

Bipolar disorder, previously known as manic depression, is a condition that affects your moods, which can swing from one extreme to another. People with bipolar disorder have episodes of depression – feeling very low and lethargic - and mania – feeling very high and overactive. Symptoms may depend on which mood the patient is experiencing. Unlike simple mood swings, each extreme episode of bipolar disorder can last for several weeks (or even longer).

Source: NHS