I keep meaning to try one some day. What I think it is, is that I have a few shows I choose with a strong male voice who narrates or talks a lot (I used to choose Mythbusters, now it’s River Monsters, lol), stuff I’ve seen many times and I just let the voices lull me to sleep. I often choose a book on tape as it’s the same effect, but if it’s a book I haven’t read recently it pulls me in instead of lulling me to sleep.