To investigate the relationship between olanzapine, melatonin, and metabolic functioning, this pilot study is evaluating 20 patients with schizophrenia, schizoaffective disorder, or bipolar disorder over 15 weeks under three experimental conditions: 1) baseline (two weeks treatment with already established antipsychotic medication other than olanzapine or clozapine), 2) six weeks treatment with olanzapine only, and 3) six weeks treatment with olanzapine and melatonin. Half of the patients will receive 0.3 mg of oral melatonin and half will receive 3.0 mg of melatonin. Nocturnal melatonin production, as estimated by assay of urinary 6-sulfatoxymelatonin(aMT6s) adjusted for creatinine, will be measured weekly. In addition, weekly measurements of weight and other metabolic indices, including waist and hip measurements, fasting glucose, serum insulin, cholesterol, triglycerides, and leptin will be taken. It is anticipated that there will be an olanzapine-induced decrease in melatonin production. Furthermore, it is expected that the decrease in melatonin production associated with olanzapine treatment will be reversed by administration of melatonin with olanzapine.