Background. It is believed that the severely disturbed night-time sleep architecture after surgery is associated with increased cardiovascular morbidity with rebound of rapid eye movement (REM). The daytime sleep pattern of patients after major general surgery has not been investigated before. We decided to study the circadian distribution of sleep phases before and after surgery. Methods. Eleven patients undergoing elective major abdominal surgery were included in the study. Continuous ambulatory polysomnographic monitoring was made 24 h before surgery and 36 h after surgery, thus including two nights after operation. Sleep was scored independently by two blinded observers and the recordings were reported as awake, light sleep (LS, stages I and II), slow wave sleep (SWS, stages III and IV), and REM sleep. Results. There was significantly increased REM sleep (P=0.046), LS (P=0.020), and reduced time awake (P=0.016) in the postoperative daytime period compared with the preoperative daytime period. Five patients had REM sleep during the daytime after surgery. Three of these patients did not have REM sleep during the preceding postoperative night. There was significantly reduced night-time REM sleep for two nights after surgery compared with before surgery (P=0.001). Conclusions. Patients have significantly increased REM sleep, LS, and reduced time awake during the daytime period after surgery compared with before surgery. Disturbances in the circadian regulation of the sleep-wake cycle may be involved in the development of postoperative sleep disturbances.