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   Major depressive disorder: a mood disorder having a clinical course involving one or more episodes of serious psychological depression lasting two or more weeks each with no intervening episodes of mania.

   Major depression is manifested by a combination of symptoms that interfere with the ability to work, study, sleep, eat, and enjoy once pleasurable activities. Such a disabling episode of depression may occur only once but more commonly occurs several times in a lifetime.

   Dysthymic disorder, also called dysthymia, involves long-term (two years or longer) less severe symptoms that do not disable, but keep one from functioning normally or from feeling good. Many people with dysthymia also experience major depressive episodes at some time in their lives

   Seasonal affective disorder (SAD), is characterized by the onset of a depressive illness during the winter months, when there is less natural sunlight. The depression generally lifts during spring and summer. SAD may be effectively treated with light therapy, but nearly half of those with SAD do not respond to light therapy alone. Antidepressant medication and psychotherapy can reduce SAD symptoms, either alone or in combination with light therapy.

   Bipolar disorder, also called manic-depressive illness is not as prevalent as major depression or dysthymia, and characterized by cycling mood changes: severe highs (mania) and lows (depression).