Also known as manic-depressive disorder because the patient often experiences dramatic and rapid mood changes, from mania to depression.
Mania: defined by uncharacteristically "high" behavior and the belief of the patient that he or she is unable to be brought down from their elevated state of mind. Behaviors may include
Increased sex drive
Overestimating one's abilities (ex. holding the belief that he or she is literally invincible)
Feeling "wired", insomnia, restlessness, anxiety, irritability
Rapid thought processes
Abuse of drugs and alcohol
Depression: (see Depression for more information)
Lack of interest in hobbies, daily life, sex
Problems with sleep; excessive sleepiness, fatigue or insomnia
Memory problems, difficulty concentrating/focusing
Feeling helpless or hopeless
Feeling numb or empty
Social withdrawal, anxiety/irritability
Thoughts of suicide, death or suicide attempts
Unintentional weight gain or loss
Restlessness, anxiety, irritability
Severe episodes of either mania or depression may be accompanied by psychosis which involves hallucinations and delusions. Some people with bipolar disorder are mistakenly diagnosed with schizophrenia (hyperlink) because psychosis during mania can lead a person to believe that he or she is famous or invincible, while psychosis during depression may lead a person to believe that he or she has committed a felony or has been reduced to a state of poverty.
Some people may experience what is known as a mixed bipolar state where they would be expected to experience a combination of the symptoms of both depression and mania. For example, a person could possess suicidal thoughts while simultaneously experiencing an increase in energy and restlessness.
Treatment: There is currently no cure for bipolar disorder, although the symptoms can be easily managed by a drug regimen consisting of mood stabilizers, the most popular of which is Lithium, and sometimes an anti-depressant to help patients cope with depressive episodes.
For more information see the NIMH page on Bipolar Disorder