Results of the study, which included 928 Icelandic men, suggest men who have higher levels of melatonin may have a lower risk of developing prostate cancer, said lead author Sarah Markt, a doctoral candidate in the department of epidemiology at Harvard School of Public Health.
"Men who had higher levels of melatonin had a 75 percent reduced risk for developing advanced prostate cancer compared with men who had lower melatonin. The risk was especially reduced when it came to advanced disease," Markt said.
The research was scheduled for presentation Sunday at an American Association for Cancer Research meeting focused on prostate cancer, held in San Diego.
Melatonin is believed to be connected with the body's circadian rhythms, Markt said, although it isn't necessarily produced during sleep. The hormone is secreted by the pineal gland during the night, and represents the biochemical signal of darkness, she added.
She was interested in studying a potential link between melatonin levels and prostate cancer because a number of other studies have suggested that low melatonin and disrupted sleep can be related to health problems, including a potential risk for cancer.
"In experimental studies -- animal studies and prostate cancer cell lines -- it's been shown that melatonin has an inhibiting effect on prostate tumor growth," Markt said.
She and her colleagues analyzed data from a 2002 to 2009 study of Icelandic men. The investigators used urine samples from participants to measure melatonin levels and linked them to cancer and death registries. The men also answered a questionnaire about sleep patterns.
One in seven men reported problems falling asleep, one in five men said they had problems staying asleep, and about one in three reported taking medication to help them sleep, the study found. Men with sleep troubles had significantly lower levels of the chemical 6-sulfatoxymelatonin -- a breakdown product of melatonin -- compared to men who reported no sleeping issues.
Of participants, 111 men were diagnosed with prostate cancer, including 24 with advanced prostate cancer, researchers learned.
Although the study found an association between melatonin levels and prostate cancer risk, it did not prove a cause-and-effect relationship, and experts noted that other factors might be involved.
June Chan, a professor of epidemiology, biostatistics and urology at the University of California, San Francisco, who was not involved with the study, said the findings are provocative and merit further confirmation and study.
"The association between melatonin and cancer has been observed in other cancer types as well," Chan said, adding that reports date back as far as 1970, and include breast cancer studies.
She added that research would need to rule out other potential risk factors that track with melatonin, such as exercise, and vitamin D levels. "I think these data are pretty early to suggest that men should take melatonin supplements solely for prevention of prostate cancer," Chan said.
Dr. Stephen Freedland, an associate professor of urology and pathology at Duke University, said the study concept is plausible, but he added that the fact that the men in the study come from a place with light extremes might also affect results.
"One confounding factor is that these are men in Iceland," Freedland said. "They spend practically six months of the year with no sunlight and six months with sunlight. What time of year was this done? There's actually data that suggest sunlight may be good for prostate cancer."
Study author Markt agreed that the results are not conclusive. "Prospective studies to investigate the interplay between sleep duration, sleep disturbance and melatonin levels on the risk for prostate cancer are needed," she said.
Because this study was presented at a medical meeting, the data and conclusions should be viewed as preliminary until published in a peer-reviewed journal.
The U.S. National Library of Medicine has more about melatonin.