Mood disorders are mental disorders characterized by periods of depression, sometimes alternating with periods of elevated mood. While many people go through sad or elated moods from time to time, people with mood disorders suffer from severe or prolonged mood states that disrupt their daily functioning. Among the general mood disorders classified in the fourth edition (1994) of the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) are major depressive disorder, bipolar disorder, and dysthymia.

In classifying and diagnosing mood disorders, doctors determine if the mood disorder is unipolar or bipolar. When only one extreme in mood (the depressed state) is experienced, the depression is called unipolar. Major depression refers to a single severe period of depression, marked by negative or hopeless thoughts and physical symptoms like fatigue. In major depressive disorder, some patients have isolated episodes of depression. In between these episodes, the patient does not...