[frereubu]

I see what you mean in terms of movement, but lying down is also not a natural position for a human body except for the duration of sleep. I mean, you might be able to avoid bed sores, but you're not using your core muscles in the way that the body is evolved for and your joints aren't being used in a way that will keep them in tune. It's a bit like astronauts losing a lot of general body tone and having to go through quite a bit of conditioning when they return to earth.