If there is a street light of porch light which shines into the window of that room, use dark curtains to exclude the light. You may also remove the poinsettia and Christmas cactus plants and place them in a dark closet each night, but remember to bring them back into the light each morning. It will also not harm them to forget and leave them in the closet for a day or two, but don't leave them there constantly. They need light to develop their flowers, just no more than ten to twelve hours each day. Continue to water them as they dry, but don't apply much fertilizer until after they have bloomed and growth resumes in the spring.
Temperature also influences flower development.
Cool temperatures near 60 degrees each night are ideal. Slightly warmer or cooler temperatures will not be a problem.