Ever see the movie
Amadeus? Where Mozart plays the piano while lying down and he's rockin those keys! I thought that was so cool that I tried it, myself. Man, I suck.

First, the muscles that held my arms up while lying down were weak and I couldn't hold them up for long amounts of time. Second, I couldn't see the keys so I had to pluck them out to find the right notes. Third, since I was upside down, the right hand had to take the place of the left and vice versa.
Lastly, be careful getting up. My forehead hurts.