PURPOSE: Our intention was to assess the effectiveness of preoperative oral melatonin medication on sedation, sleep quality, and postoperative analgesia in patients undergoing elective prostatectomy. METHODS: Fifty-two ASA I-II patients undergoing elective prostatectomy were included in this study, randomly divided into two groups. Patients received an oral placebo (n = 26) or 6 mg melatonin (n = 26) the night before and 1 h before surgery. All patients received a standard anesthetic protocol. At the end of surgery, all patients received tramadol i.v. via a PCA device. Extubation time, intraoperative fentanyl consumption, and recovery time were assessed at the end of the operation. Pain scores, tramadol consumption, and sedation scores were assessed at 1, 2, 4, 6, 12, 18, and 24 h postoperatively, and sleep quality and subjective analgesic efficacy were assessed at 24 h after surgery. RESULTS: There were no significant differences in demographic data between the groups. Extubation time and recovery time from anesthesia were significantly longer in the melatonin group (P < 0.05). Intraoperative fentanyl usage, pain scores, and tramadol consumption were significantly lower in the melatonin group (P < 0.05). The postoperative sleep quality of patients was significantly better in the melatonin group than in the control group (P < 0.05). Postoperative VAS of pain was significantly lower in the melatonin group compared with the control group at 1, 2, 4, 6, 12, 18, and 24 h postoperatively (P < 0.05). Subjective analgesic efficacy of patients was significantly different between groups (P < 0.05). The sedation scores were significantly higher in the melatonin group than in the control group at 1 h and 2 h after surgery (P < 0.05). CONCLUSIONS: Preoperative oral melatonin administration decreased pain scores and tramadol consumption and enhanced sleep quality, sedation scores, and subjective analgesic efficacy during the postoperative period.