I can't add much to her words here and here, but can agree with them wholeheartedly. I often leave the light off when going to bed - I sleep badly and read somewhere that harsh light can exacerbate this. It is so lovely, especially when the moon is full, to move through the varied grays and golds of candlelight and dusk in the house, to really see the varied gradations of light as the earth spins away from the sun beyond the horizon, to marvel at the twinkling red of the TV mast 13 miles away in Heathfield and to wonder about the thoughts of those souls in the plane, whose lights are sparkling in the night sky. All of this disappears when the light is turned on - all that can be seen is reflections in the room, not the great space of sky outside.
Mind you, I do smile at myself as I open the bathroom door a crack to let enough light in from landing to enable me to apply the toothpaste to the brush rather than my thumb.