Falling asleep on your back isnt an absolute rule that you have to follow. Its just something that ive noticed helps me fall asleep faster.

If you can fall asleep faster in another position by all means go for it.
Oh, I thought you wanted to make it hard to fall asleep, so your mind is more awake or something. That's why I try to start WILDing from my back. I usually can't fall asleep though and I shift to my side after about 30 or 40 minutes