I am not quite sure, but I do know that when the body enters the deepest form of sleep the spinal cord disconnects from the body to brain rendering the body temporarily paralysed,(this is why some people who wake during this state are unable to move), this may cause greater activity in the brain.
That's probably a personal thing. I have the most intense dreams when I am sleeping my lightest. I always sleep on my stomach, in the same position (sigh), so my sleeping position has no effect on my dreams.