Objectives : Daytime drowsiness or sedation and changes in night sleep are commonly seen in patients treated with clozapine. There is, however, very limited information on their degree and nature during the course of treatment. The purpose of this study was to understand the sleep patterns in chronic schizophrenic patients with clozapine treatment over a period of 24 weeks. Method : The sleep pattern was evaluated using a set of 5-point scale questionnaire, to record subjective impressions of the night sleep induction, maintenance and quality, and daytime drowsiness and fatigue. In addition, unusual experiences associated with night sleep were recorded. The sleep questionnaire was repeatedly administered at baseline and at 1, 2, 4, 8, 12 and 24 weeks of drug treatment. At present, data on 12 patients has been collected. Results : All the components of night sleep were significantly improved in the 1st through the 12th week after treatment with clozapine. Daytime drowsiness was significantly higher in the 1st to the 2nd week after the treatment and fatigue was also significantly higher in the 1st to the 4th week after the treatment. Eight patients experienced noticeable increases in salivation during night sleep, and of these, one also reported frequent nocturnal urination and even enuresis. However, all these adverse factors did not affect the major sleep patterns. Conclusions : These findings suggest that the beneficial effects of clozapine on night sleep might last much longer than the undesirable effect of daytime drowsiness and fatigue. In other words, tolerance of the hypnotic action of clozapine might develop late and tolerance of the daytime drowsiness and fatigue might be evident earlier.