Inclusion Criteria:

• Age 18-69 years.
• Diagnosed with ulcerative colitis and currently experiencing a flare up of mild-moderate severity according to Mayo Scoring system (>=4 and <=10).
• Has a regular GI physician(s).
• Availability for 14 weeks after enrolling in the study.
• Female patients of child bearing potential must be surgically sterile or have a negative urine pregnancy test and practice acceptable contraception (i.e., abstinence, oral, intramuscular, or implanted hormonal contraception, 2-barrier methods such as condom, diaphragm or spermicide).

Exclusion Criteria:

• Night shift workers; otherwise, inability to attend morning study visits.
• Current severe colitis according to the Mayo Scoring System (>10) or requiring hospital admission, or current very mild colitis (<4).
• Current supplemental intake of melatonin or currently using tobacco products.
• Proctitis or Crohn's disease
• Previous gastrectomy or small or large bowel resection
• Malabsorption syndrome (e.g., pancreatic insufficiency).
• History of large bowel resection for any reason.
• Diagnosed narcotic or alcohol dependence.
• On renal dialysis.
• Active liver disease (a chronic condition or under current therapy)
• Seizure disorders
• Immune system disorders
• Any history of hypo- or hyperparathyroidism.
• Unable to be off aspirin for 7 days.
• Use of systemic or rectal steroids in the past 8 weeks.
• Have a history of allergy or hypersensitivity to melatonin.
• History of bleeding disorder or current use of anticoagulant medication.
• Current use of calcium channel blockers, anticonvulsants, sedatives, hypnotics, or psychotropic medications.
• Women who are pregnant, breast-feeding, attempting conception, or planning to attempt conception over the following 6 months, or are currently taking hormone replacement therapy (HRT).