The advice I got at our hospital physiotherapy class was that you should try and avoid it. They said if you wake up and you are on your back, don't freak out, just roll to your side, and that chances are you woke up because it wasn't quite right.

They made a very strong point that just because you don't feel dizzy and light headed lying flat on your back it doesn't mean that the blood flow isn't restricted. For you to feel sick, the blood flow has to be significantly restricted. But any sort of restriction for an extended period of time will effect bubs (not necessarily in a disastrous way) even if you are unaware of it.

Getting around it is pretty easy. Just prop yourself up, or try putting a pillow under one side so you can still lay on your back, but your not flat on your back.

Like the other girls said a body pillow is the best investment you can make!!! I got mine when I was about 16 weeks and love it to death!