When it's dark out, I sleep with colored Xmas lights on-they create cheerful warm glow.
When going to bed during the day (am a night owl, and thus frequenly a day sleeper), I turn those off & instead there's daylight shining in-doesn't interfere with my falling asleep, though.
Every once in awhile I go to sleep in total darkness (but usually need some little bit of light so I can see the cable/tv remote control). If I know I'll be falling asleep soon, I turn off the tv monitor & leave on the cable & stereo (which display a few small colored lights, not enough by which to see), so I can listen to music or shows, w/o flashing pictures on screen distracting.
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*"I don't know what it is, but I know what it isn't."*