I definitely prefer total darkness. Currently my bedroom is very dark but the alarm clock is bright enough that I cover it. When I’ve lived in brighter homes, such as in a more urban environment without curtains, I’ve adjusted to sleeping in a dusky amount of ambient light, but in general I always sleep much better in total darkness. My idea of vacation used to include a bedroom that was pitch black at night…aah, sweet relief! Once, when I was a kid, the Christmas tree was in my bedroom because that was the corner window visible from the sidewalk. My sister was in town for the holiday and sharing my room. The tree crashed to the floor in the middle of the night but I didn’t actually wake up until someone ran in and flipped on the lights.
Recently someone told me about study results reporting that sleeping with too much light can exacerbate depression. Also, all living creatures, even plants, have a circadian rhythm that is affected by light among other things. So I guess that might support the depression statement. From Wikipedia (link): “Although circadian rhythms are endogenous (“built-in”, self-sustained), they are adjusted (entrained) to the local environment by external cues called zeitgebers, commonly the most important of which is daylight.”
However, “Lighting levels that affect the circadian rhythm in humans are higher than the levels usually used in artificial lighting in homes. According to some researchers the illumination intensity that excites the circadian system has to reach up to 1000 lux striking the retina.”