I know, by observing people and from my own experience (you can try it yourself while standing, but not too consciously and in a loose way) that they, while standing, tend to do this on one leg. Just observe yourself and other people. Many birds do this as well. Like the stork:
The reason for this is to reduce heat loss:
Why do some birds stand around on one leg? A. One widespread theory among bird experts involves heat conservation. Herons, hawks, geese, ducks, and gulls — birds both short-legged and long-legged — are on the Audubon Society list of those that tuck up one leg under their feathers and balance on the other.
The question remains: why do people too tend to do this? We don't pull up our entire leg though. Many times I've seen people standing on one leg, their other leg crossed over the other.
Leaning on one leg Leaning more on one leg while standing, sometimes referred to as “hanging on one hip”, can feel comfortable, especially if you've been standing for a while. Instead of using your buttocks and core muscles to keep you upright, you place excessive pressure on one side of your lower back and hip. Mar 26, 2016