Doesn't surprise me one bit ...
As I've traveled for the past decade, I've learned to sleep upright in a chair. It requires good, solid posture with your back and head flush. It takes some getting used to, as we've programmed ourselves long ago to slouche to defy gravity. It requires you to balance yourself against gravity using your natural posture.
Although I would not remotely attempt to sleep standing up. Now that would be impossible to do for most people, including myself. But sleeping while sitting up straight is an art that is not that difficult to do.
