facing left
On your left side. Whether that’s “facing left” or “facing right” depends on whether you’re comparing it to being on your front or on your back. Personally, I instinctively compared it to front, which would mean being on your left is facing to the right.
So the way to be clear and unambiguous is to say which side of your body you’re referring to.
It’s your left, not the left of the dude living in your attic peeping down at you.
Yes, but to know what is your left, one first needs to establish what is their forward. If you were previously on your front (which is itself not an uncommon sleeping position), “turn to face left” will put them lying on their right side.
This stuff really isn’t rocket science. I’m genuinely surprised to be getting push back here. If the goal is to tell people to lay on their left side, that’s what should be said. Not “facing left”, which doesn’t convey the same meaning.
But your left hand is always the left one. It’s relative to you, not to your direction.