I used this for a while, combined with (usually once, before the breathing technique) a sort of "bed yoga" thing, tightening muscles and then relaxing them until I start to feel at-one with the bed, an unusual feeling I can't much describe.
I now don't have to consciously do the breathing counts or the exercise; my brain and body can find their way to that same place with just careful, slow breathing with the long exchale, and a focus on that feeling.
But on its own, the 4-7-8 trick is enormously impressive. You don't even have to trust that it works. Though no caffeine after 2pm probably helps too.