On Earth, gravity pulls down on us. In the standing position, that means our head is pressing on our neck, our neck is pressing on our torso, our torso is pressing on our legs, and our legs are pressing on our feet. We lie down, partly, because in doing so we spread the load across our body, taking a lot of stress off of the lower parts of our body.
We also lie down because it leaves us in a stable position, so we don’t have to worry about losing our balance and falling, once we are asleep and no longer actively maintaining our balance.
In an orbiting spacecraft, we are in free fall, so we experience weightlessness. All of those red arrows (loads from gravity) disappear. The body now experiences no change in loading from the vertical to the horizontal position. Both are equally stable and both feel the same.
So, sleeping is done in whatever position best fits the available room.
Credit : Quora
Picture Credit : Google