Researchers at Ohio State University made this discovery by messing with the sleep cycles of mice. One group got to sleep in darkness, while the other had to snooze in roughly the equivalent of a cheap strip club. Not only did the mice exposed to light gain weight but they actually increased their body mass a full 50 percent. In eight weeks. Oh, and they also showed signs of impaired glucose tolerance, which of course is science-speak for two heaping scoops of diabetes.
There are a few possible reasons behind the phenomenon. One theory says it could be those pesky melatonin levels (which also regulate metabolism) again. Alternately, the nocturnal lights could mess with clock genes (which control when animals are active and eating). Whatever the cause, the light messed with their circadian rhythm, they behaved more nocturnally, and they started feeding at times they normally wouldn't. Not eating more than usual, just at different times. Of course, when the experiment was conducted with restricted access to food, nobody gained a pound. So there you go, easy solution: just make sure there's never any food in your house after sundown and you're golden.
Actually, let us know when you've got that ready. We've been trying to unload these Mogwais for months.