Background and aims: Melatonin, a sleep promoting agent, is involved in the regulation of gastrointestinal motility and sensation. We aimed to determine if melatonin was effective in improving bowel symptoms and sleep disturbances in irritable bowel syndrome (IBS) patients with sleep disturbance. Methods: Forty IBS p atients (aged 20-64 years;24 female)with sleep disturbances were randomly assig ned to receive either melatonin 3 mg (n = 20) or matching placebo (n = 20) at be dtime for two weeks. Immediately before and after the treatment, subjects comple ted bowel, sleep, and psychological questionnaires, and underwent rectal manomet ry and overnight polysomnography. Results: Compared with placebo, melatonin take n for two weeks significantly decreased mean abdominal pain score (2.35 v 0.70;p < 0.001) and increased mean rectal pain threshold (8.9 v -1.2 mm Hg; p < 0.01) . Bloating, stool type, stool frequency, and anxiety and depression scores did n ot significantly differ after treatment in both groups. Data from sleep question naires and polysomnography showed that the two week course of melatonin did not influence sleep parameters, including total sleep time, sleep latency, sleep eff iciency, sleep onset latency, arousals, duration of stages 1-4, rapid eye movem ent (REM) sleep, and REM onset latency. Conclusions: Administration of melatonin 3 mg at bedtime for two weeks significantly attenuated abdominal pain and reduc ed rectal pain sensitivity without improvements in sleep disturbance or psycholo gical distress. The findings suggest that the beneficial effects of melatonin on abdominal pain in IBS patients with sleep disturbances are independent of its a ction on sleep disturbances or psychological profiles. Background and aims: Melatonin, a sleep promoting agent, is involved in the regulation of gastrointestinal motility and sensation. We aimed to determine if melatonin was effective in improving bowel symptoms and sleep disturbances in irritable bowel syndrome (IBS) patients with sleep disturbance. Methods : Forty IBS p atients (aged 20-64 years; 24 female) with sleep disturbances were randomly assig ned to receive either melatonin 3 mg (n = 20) or matching placebo (n = 20) at be dtime for two weeks. Immediately before and after the treatment, subjects comple ted bowel, sleep, and psychological questionnaires, and underwent rectal manomet ry and overnight polysomnography. Results: Compared with placebo, melatonin take n for two weeks significantly reduced mean abdominal pain score (2.35 v 0.70; p < 0.001) and increased mean rectal pain threshold (8.9 v -1.2 mm Hg; p <0.01). Bloating, stool type, stool frequency, and anxiety and depression scores did n ot significantly differ after treatm ent from both groups. Data from sleep question naires and polysomnography showed that the two week course of melatonin did not affect sleep parameters, including total sleep time, sleep latency, sleep efficiency, sleep onset latency, arousals, duration of stages 1-4 Conclusions: Administration of melatonin 3 mg at bedtime for two weeks significantly attenuated abdominal pain and reduc ed rectal pain sensitivity without improvements in sleep disturbance or psycholo gical distress. The findings suggest that the beneficial effects of melatonin on abdominal pain in IBS patients with sleep disturbances are independent of its a ction on sleep disturbances or psychological profiles.