Lacking a medical study that might bring 'fact' to this thread I don't feel guilty in adding that imo lie-flat beds are always good for one's bones and well-being. A sitting position, especially strapped-in static, is a bad idea even for an hour, and few take the airlines' advice to move around in the cabin (and I can't imagine that they really want everyone to do that).

And if you find yourself in a bed-able seat, tired and beat up for whatever reason, it feels like heaven.