In pilot experiments with 24 depressed patients, a single 1-hr. exposure to bright (1,000-2,000 lux) light from 2 hrs. to 1 hr. before the patient's usual time of arising reduced depressive symptoms. Results after exposure to bright white light were significantly better than after 1-hr. exposure to dim red light. These results are partially explained by the phase advance theory of depression and by a critical interval hypothesis utilizing a model analogous to rodent phtoperiodic responses. Thus, the suprachiasmatic nucleus, the pineal, and melatonin may be involved. To confirm these results with a 7-day duration of treatment, 36 inpatients with Major Depressive Disorders will be studied. After 2-day baseline, each patient will be treated with 1 week of bright light and 1 week of dim red light (in counterbalanced orders). Follow-up ratings will be obtained for an additional week. The light exposures will be given during a 5:00 A.M. - 6:00 A.M. awakening. EEG sleep recordings will be obtained throughout baseline, the light exposures and 2 nights of follow-up Hamilton and Beck depression ratings and a circadian mood self-rating will be obtained daily. Urine will be collected for 6-OH-melatonin-sulfate assays on 3 nights during each light treatment. This clinical trial is a step toward developing bright light as a clinical treatment for depression.