In this study, two females, siblings who exhibited a non-24 h sleep-wake rhythm (non-24 h) at home were observed. However, they showed a delayed sleep phase syndrome (DSPS) immediately after admission to Kurume University Hospital. Melatonin (3 mg) was commenced following chronotherapy and this improved their sleep-wake rhythm. Polysomnography (PSG) showed decreased sleep latency and increased sleep stage. In these cases, the involvement of environmental factors was strongly suggested for the sleep-wake rhythm abnormalities as well as familial factors.