Melatonin is the hormone produced by your brain when it gets dark - it tells your brain it's time to sleep. This study analyzed levels of a melatonin metabolite in the urine of older men from Iceland. The results showed that melatonin metabolite levels were lowest in those that reported sleep aid use, trouble falling asleep, or trouble staying asleep. And those men that had the highest levels of the metabolite had a 75% decreased risk of advanced prostate cancer.
The article does not discuss possible mechanisms, and more studies are needed before melatonin is recommended to ward off prostate cancer.