This thread may be old but I was searching the web for related topics because this happens to me all of the time.

I was actually more worried about it that anything I have a preference of sleeping on one side but notice a drastic change in dreams when sleeping on either my back or my opposite side. I was worried that pooling of blood on one side may damage that side of the brain or perhaps the lack of blood on the opposite side could be damaging.

Maybe we should all rotate the positions that we sleep in.