getting back to sleep

Years ago I read a method to get to sleep in a book on lucid dreaming (where you can figure out you're having a dream, and control the way it goes - and even decide to fly in your dreams).

You imagine you're at a door opening onto steps leading down to a dark basement. At each of 20 steps going down, as it gets darker and darker, say "20 - I'm dreaming", "19 - I'm dreaming", etc. until you get to the bottom. You may have to do this over and over for awhile, but it always works for me.