I had that too, I haven't slept on my back in 10 years. In my dream it seemed to me that I wanted to wake up, sometimes I even woke up half and I couldn't move. It was a terrible feeling, really scary. I have a well-known doctor and I know that he explained to me at the time that it is because it is on your back that the brain is in such a position that this will happen to you. But only someone has it. As I have been writing since, I have only slept on my side for about 10 years and I no longer had it.