"There is a strong genetic component determining whether a person is an evening or a morning type in their activity times," said Penn State psychology professor Frederick Brown. "Everyone has an inborn basic biological rhythm -- also called a circadian rhythm. Across the population, that rhythm averages about 24.1 hours long."
During that cycle, many "phase relationships" play themselves out. One is body temperature. Average normal body temperature is about 98.6 degrees F. "Our normal temperature is at its lowest phase about two hours before we waken," Brown said. "In some individuals, it drops to nearly 96 degrees. Every day our body temperature slowly increases until its highest phase may top out at well above 99. Later in the evening and into the night, our temperature drifts slowly downward again to its lowest phase."Read more at: http://phys.org/news84126028.html#jCp
Might that genetic component also be connected to what time of day someone is born? This is my personal philosophy. But I'm not really sure: Were you born in the morning because you are genetically disposed to mornings or are you genetically disposed to mornings because you were born early in the morning?