I have always found just closing my eyes makes it seem dark. Try it. Dark inside, right? Most people have no problems taking an afternoon nap in a sun lite room. Most of the years we lived in rural Alaska, we tended to ignore our watches. When you are tired, go to bed. When rested, get up. Can make for a problem with a family being on different sleep schedules.
Light or dark in the bedroom, if you are tired, you will go to sleep. If not get up and do something else.
As a disclaimer, my wife doesn't agree with my philosophy of just closing the eyes.