[Jorg]

When lying on your back there's still the small small small risk of swallowing your tongue if you should blackout. One of the reasons why you should always put an unconsciousness person on his side an not on his back.

I think sitting in a sofa is the best position. Just hanging back with no risk of falling on the floor. Extra advantage over lying on the bed is that with sitting in a sofa your abdominal muscles are relaxed so it's become much easier to go through your contraction then with a straight body. One of the reasons why you see a lot of freedivers going from a straight floating position to a standing position in the end.