Melatonin is a hormone naturally produced by the body. It has been found, and is often used, in helping to promote sleep and to improve the immune system, helping to protect us from certain diseases. Some evidence shows that melatonin may also help in preventing and reducing the size of tumors.
A Review of the Research
In a study published in the May/June 2008 issue of the Medical Science Journal, mice were first treated with benzo(a)pyrene, a cancer-causing chemical. Some of the mice were then treated with a topical solution of melatonin. Those that received the melatonin treatment were less likely to develop tumors and had fewer lesions than the mice that did not receive this treatment.
Dr. Russel Reiter, in an article on AnnieAppleseedProject.org states that the evidence that melatonin can help protect against tumors is strong but it is not a miracle cure.  More research is needed to understand the relationship between melatonin and skin cancer and to determine how this information can be used to help patients with skin cancer but the results of studies are very encouraging.
A previous study, published in the April 2003 issue of Melanoma Research, found that large doses of melatonin can kill melanoma cells and may be helpful in fighting cancer.
The Journal of Pineal Research published the results of an experiment conducted in 2010 showing that melatonin prevented the growth of cancerous cells. In this experiment, cancer cells were grown in a laboratory in a dish and were then treated with melatonin.
A study as far back as 1983 shows the relationship between melatonin and tumors. A research project reported in the journal Endocrinology examined hamsters after the pineal gland, the gland where melatonin in produced, was removed. Those hamsters that no longer had a pineal gland had larger tumors than those who still had pineal glands.
While a strong correlation between melatonin and melanoma has been found in a number of research projects, you shouldn't go out and immediately begin taking large quantities of melatonin supplements. Many of these experiments and studies have used a topical preparation which is applied directly to the skin cancer or skin lesion.
Before taking any supplements, you should speak with your doctor. Some supplements can cause side effects or interfere with any medication you are currently taking. You and your doctor can determine if melatonin is a good idea for you and help determine what the best dosage is.
"Melatonin Decreases Cell Proliferation and Induces Melanogenesis in Human Melanoma SK-MEL-1 Cells," 2010, August, Javier Cabrera, Journal of Pineal Research
"On the Role of Melatonin in Skin Physiology and Pathology," 2005, July 27, A. Slominski et al, Endocrine Journal
"Pharmacological Action of High Doses of Melatonin on B16 Murine Melanoma Cells Depends on Cell Number at Time of Exposure," 2003, April, L.K. Yerneni, S. Jayaraman, Melanoma Research
"Photoperiodic Control of Melanoma Growth in Hamsters: Influence of Pinealectomy and Melatonin," 1983, August, Lawrence R. Stanberry et al, Endocrinology
 "Study in Mice Shows Melatonin May Offer Preventive Effects," Date Unknown, Staff Writer, AnnieAppleseedProject.org