Looks like there has been significant research on chronotype (A person's preference for daytime vs. nighttime activities). One study showed "About 1/10 people are actually hard-wired... to be early risers" and "2/10 people are what are known as night owls". So there maybe some proof to your genetics theory. People maybe hardwired to reach peak functionality at different times.
Which would make sense in evolutionary terms. You want someone in your group to be awake at night as a watch. Its also very helpful to have very early morning risers, to prepare stuff for the group.
http://www.kwollity.com/2010/03/are-you-a-late-or-early-chro... http://en.wikipedia.org/wiki/Chronotype http://www.roundtheclocksystems.com/methods5_ctest.html