Another abstract on melatonin and cancer but the real bombshell for me is this
ABSTRACT
In further experiments, Blask's team determined that melatonin blocks cancer cells' metabolism of linoleic acid, a polyunsaturated fat that's abundant in food. The same team had previously shown that 13-hydroxyoctadecadienoic acid, the product of linoleic acid metabolism, spurs cancer cells to divide.
Link
http://www.sciencenews.org/articles/20060107/bob9.asp