I can't recall literally laying down to sleep and dreaming within a dream, but I resonate with the link you posted, Rodrodrod. I've had experiences of dreams layered on top of one another, around 3-4 at a time, occurring at the same time - though they were not as this OP mentions 'a dream within a dream'. They were simply sequences running at the same time as one another. From what I can tell of my own experience, this requires multiple points of perception along with rapid synchronized information cataloging. In this way, [perceived] layered dreams are distinct from singular dreams.