Melatonin is the hormone produced in your brain when the sun goes down. It is the signal that says it's time for sleep soon. This article talks about a study that showed that people with low amounts of melatonin at night were twice as likely to develop type 2 diabetes.
It will be interesting to see if taking melatonin would prevent people who are at risk from developing diabetes. Research also needs to be done to figure out how and why melatonin is related to diabetes. I wonder if people with type 1 diabetes also have low melatonin levels at night.