the older folks said that if the baby keeps sleeping on one side, his face will be flatter on that side.
To me, the rule is simple.
So long as my baby sleeps well and enjoy sleeping in that position, I will let him be.
Why 'create' trouble for myself by semi-waking him up, turning him, so that his face is proportionate? *giggle* Nahhh... to me, that's asking for trouble, that is disturbing his precious sleep.
When my boys were baby....
My elder one loved to sleep on his right. Fell asleep easily (its a great thing to a tired Mommy) on that side.
My younger one loved to sleep on his tummy. Many say its dangerous. Well, Mommy did stay alert by check every now and then. But he sleeps like a log in that position, so what's that little walking in and out of the room compared to him having a great sleep. *wink*
Thus my answer to your question is, let him sleep. Even though its few hours. My elder one, didn't have a flat right side, nor did he have brain/mental problem. So no side effects.