Atypical antipsychotic medications, such as olanzapine, cause metabolic side effects, including weight gain, extra fat around the middle of the body, high blood sugar, and high cholesterol. One of the mechanisms by which these medications may cause these effects is by reducing plasma melatonin. This study is a pilot project to evaluate 1) the effect of olanzapine on melatonin secretion levels and 2) the effect of melatonin on olanzapine-induced changes in melatonin secretion in patients with schizophrenia, schizoaffective, or bipolar disorder.