On two legs when you relax you would fall over as being vertical is not stable. On four legs you have a stable base and can sleep standing up but your orientation there is still horizontal. A vertical body will attempt to restore it self to a stable horizontal state. IE if you fall asleep in your chair you will eventually face plant forward or fall over to one side. Science with Vicki HHHHHHHHHOOOOOOOOOOWWWWWWWWWWWWWWWWWWWWWLLLLLLLLLLLLLLLLLLL