Science has proven that some humans have more hair on one side of them than the other.
Some people think that it's because people sleep on one side and not on the other. (I sleep on my right side).
Which side do humans have more hair on?
I would have liked to tag this post hard-science just for laughs, but sadly this SE does not have that tag :P
EDIT: In response to AstroMax's answer, I realized that his answer could also work. Note: This riddle works for almost all species including humans :P