its fine unless you Dr tells you otherwise just sleep however is most comfortable to you (wich seems to get harder to do the bigger you get).
The Docs say not to lay on your back b/c the weight if your uterus presses in a vein/artery slowing down or cutting circulation to your baby I think.
You're supposed to lay on your left side but I also sometimes find myself sleeping on my back.
Some people sleep on their back throughout their whole pregnancy so I'm iffy on how good or bad it actually is.
I sleep however I'm comfortable and baby is still doing good :-) I'm 22 weeks 5 days