Major depression (also known as unipolar depression) is characterized by a severely depressed mood, accompanied by feelings of worthlessness and lack of pleasure and by sleep and appetite disturbances. To be characterized as major depression, the episode must last at least 2 weeks, but on average episodes last about 6 months. Both seasonal affective disorder (SAD) and bipolar disorder involve shorter, but cyclically recurring, depressive episodes. If you have a friend who experiences recurring periods of severe depression, how would you determine whether she is suffering from SAD or bipolar disorder?