I tend to sleep best in the dark, but I think most of it is how tired I am. Days in the past when I have only gotten 3 or 4 hours of sleep I could easily take a nap in the bright of day, but if I have decent sleep (7+hrs), a nap is nearly impossible. I think the light is more so the artificial light. Moonlight, campfires... I want to try sleeping with a window open to let in the light (stars/moon) in because the street lamp is at the front of the house, not the back where my window is, but I move into an apartment in AUG so I doubt it would be a long term thing. Artificial light, for me, is a killer.