Sleep Is For the Dark

"Keeping a light on at night could change your eating schedule, and the result could be extra pounds. Mice exposed to a dim light at night gained 50 percent more weight over an eight-week period than mice that slept in total darkness.

"The findings held up even when the amount of food and the physical activity of the mice were held constant, and the results could apply to people who eat meals late at night."

http://articles.mercola.com/sites/articles/archive/2010/10/28/sleeping-with-the-lights-on-could-cause-weight-gain.aspx?aid=CD1057

One of the tests reported in "Lights Out: Sleep, Sugar and Survival" noted that all it took to interrupt the sleep hormone cycle was to shine a dim light on the back of a sleeper's leg.  Sleeping in complete darkness is cheap and worth the experiment to see if you notice a difference.
I also think it's worth noting that sleeping in anything other than darkness wasn't frequently an option before about 100 years ago.