Inclusion Criteria:  Patients should have major LARS (LARS score >29). Patients should have undergone bowel continuity restoring surgery between the last 3 to 24 months. Participants should be 18 years or older. Participants must sign an informed consent form.  Exclusion Criteria:  Known allergic reaction to melatonin. Dementia as determined by mini mental state examination score (MMSE) < 24. Participation in another pharmacological intervention trial at the point of inclusion. Completed any adjuvant oncological treatment within the last three months. Ongoing oncological treatment. Rotor or Dubin-Johnson syndrome, epilepsy, systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), Parkinson's disease, spinal cord injury and multiple sclerosis. Severe liver disease defined as transaminases above 3 times the normal levels, and severe kidney disease defined as eGFR below 40 ml/min. Daily ongoing hypnotic treatment. Pre-existing medical sleep disorder diagnosed before the diagnosis of rectal cancer (i.e sleep apnea, restless legs, narcolepsy.) Work involving nightshifts. Daily alcohol consumption above 5 units of alcohol (1 unit = 12 g alcohol) Predictable poor compliance (due to pre-existing psychiatric disease, dementia or not able to read or speak sufficient Danish) Pregnant or breastfeeding. Severe, life-threatening medical condition, that implies that the patient cannot participate in the study course. (e.g. metastatic disease, local recurrence, stroke, AMI). Females not in menopause (defined as no menstruation during the last 12 months) should not be pregnant. Furthermore, reproductive females should use a secure birth control (intrauterine devices, hormonal contraceptives including - oral pills, patches, vaginal rings and injections) during the entire period of the trial.