A good cure for insomnia is to get plenty of sleep.
~ W.C. Fields
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Scientists in Singapore noted that persons with irritable bowel syndrome (IBS) often experience sleep disturbances. As melatonin is known to regulate sleep patterns in some, they wondered if it might also have favorable effects on bowel function as well.

Forty patients with IBS were invited down to the sleep lab for an investigation of the effects of melatonin on bowels and bedtime. While this gaseous group did not experience any favorable effects on sleep, within 2 weeks of consuming 3 mg. of melatonin each evening, their average abdominal pain scores decreased significantly compared to the placebo group, while their mean rectal pain threshold increased seven-fold.

Don't even ask how they measured that last parameter, just know that these subjects either didn't know what they were getting into, or they were handsomely paid.

I've tried 3 mg. of melatonin, more for its neuroprotective effects (experts believe that this supplement can decrease risk of Alzheimer's disease and Parkinson's) than any bowel benefits. While I noted a great night's sleep and increased dreaming, I felt foggy and hungover most of the following day.