Several reports suggest the role of sleep-wake rhythm in mood disorders. Sleep loss may be a possible trigger or augmenting factor in mania. In a group of 34 manic bipolar inpatients, we analyzed the correlation between night-time sleep duration and the intensity of manic symptomatology rated consecutively for 3 days with the Young Rating Scale for Mania and the Nurses' Observation Scale for Inpatient Evaluation (NOSIE). We found significant correlations between sleep duration and NOSIE cluster scores (cooperation and irritability).