Reduced melatonin production has been proposed as a likely factor in the significantly higher cancer rates in night workers,[28] and the effect of modern lighting practice on endogenous melatonin has been proposed as a contributory factor to the larger overall incidence of some cancers in the developed world.[29]
Fortunately you can buy Melatonin in pill form at your local drug store. 3mg pills are common, a doctor would recommend taking 1mg because that is closer to a natural dose. Side effects include vivid, lucid dreams...
Being the key word. I think the more likely explanation is night workers eat more junk food since all the normal places are closed. They don't grab a salad, they grab drive-thru or donuts or similar all-night crud.
Day workers do that too, sure, but not exclusively.
http://en.wikipedia.org/wiki/Melatonin