Keeping the light on at night. Keeping a light on at night could add a few inches to your waistline, according to a study published in October in the journal Proceedings of the National Academy of Sciences. Mice exposed to dim light at night gained 50 percent more weight over an eight-week period than mice that spent their nights in total darkness, even though all the mice had the same amount of food and physical activity. The results could apply to people who eat meals late at night, the researchers said.