Historians have recently come to believe that people are naturally biphasic in their sleep patterns. This means that before the dawn of civilization and electric lights, humans went to bed shortly after sundown and awoke during the middle of the night to cook, talk, read and reproduce.
They have also argued that before the Industrial Revolution,segmented sleep was the dominal form of human slumber in Western civilization, drawing evidence from documents from the ancient, medieval and modern world. A case has been made that maintaining such a sleep pattern may be important in regulating stress and all kinds of body functions related to hormones.
Now you know why it is so hard to drag yourself out of bed. You are meant to get up long before morning! (And of course, take a natural short nap in the afternoon).