Study aim: Effect of preoperative oral Melatonin on pain intensity after Cesarean section Design: This study is a double-blind randomized, controlled clinical trial (Phase 3). 204 patients are assigned to three groups by blocking method. The randomization process will be done with Random Allocation Software. Settings and conduct: The aim of this study was to determine the effect of preoperative Melatonin on postoperative pain intensity in patients undergoing elective cesarean section referred to Imam Khomeini hospital in Sari. Patients are divided into three groups: group A will receive a 5 mg Melatonin tablet, group B will receive a 10 mg Melatonin tablet, and group C will receive a placebo one hour before surgery. Patients and researchers are unaware of group placement. In the operating room, all patients have the same spinal anesthesia under the same anesthesia protocol. Pain intensity, nausea, vomiting, itching and headache in the study groups are evaluated and recorded 2, 6, 12 and 24 hours after surgery. In addition, the patient's first request for postoperative analgesia, the patient's opioid intake within 24 hours of surgery, and the time of the first bed rest are assessed and recorded by the anesthesiologist. It is noteworthy that the placebo required for the study was made of starch and the same color and shape of Melatonin in Sari School of Pharmacy. Participants/Inclusion and exclusion criteria: Inclusion criteria: Patients who completed the informed consent form, Lack of sensitivity to Melatonin. Exclusion criteria: Prolongation cesarean section (more than 1.5 hours), third cesarean section (CS III) and higher. Intervention groups: Group A: One 5 mg melatonin tablet (made by Nature Made, USA) Group B: One 10 mg melatonin tablet (made by Nature Made, USA) Group C: Placebo one hour before surgery Main outcome variables: Pain intensity and amount of drugs using