Analgesic Effect of Melatonin and Vitamin C Administration, Alone or In Combination in Major Abdominal Surgery.

The goal of this clinical trial is to compare the analgesic effect of melatonin, and vitamin C and their combination in patients aged 18-60 years old who will undergo major abdominal surgery with mid-line incision within an expected time of surgery from 3 to 5 hours e.g. radical cystectomy, radical prostatectomy, colectomy, sigmoidectomy, splenectomy, and CBD exploration through mid-line incision. This study hypothesizes that using both vitamin C and melatonin together as adjuvants will cause: * More reduction in total morphine consumption in the first 24 hours postoperatively * More reduction of the incidence of chronic post-surgical pain, than using each adjuvant alone. Participants will be allocated into three equal groups: Melatonin (M group) and vitamin C (V group), and Melatonin and vitamin C (VM group). Two hours before surgery all patients will receive the study drugs orally and will be continued for 3 days postoperative at the same time of the first administration; 10mg of melatonin for M group (Melatonin 10 mg - Puritan's Pride premium company), 1gm of vitamin C for V group (Sanso C 1000 mg - AUG pharma company), and 10mg of melatonin and 1gm of vitamin C for VM group. Researchers will compare: * The effect of melatonin, vitamin C, and their combination on postoperative opioid consumption * The severity of postoperative pain, using the Numerical Rating Scale (NRS) * Patients' satisfaction with a three-point scale * The time of the first requirement for rescue analgesia * The effect on the incidence of chronic post-surgical pain.