In the study where a bunch of people lived underground so as not to be influenced by natural light their bodies synchronized to a schedule a few minutes short of 24 hours. Is there any relation to the fact that thousands of years ago when human's first emerged the earth's days were slightly quicker. The earth's rotation is slowing down. The need for sleep is embedded in us and cannot be changed so this could be why the natural human sleep cycle is shorter then our current 24 hour day.

Just an idea.