nah : 11.2012 : morningness (Unordered)

Your natural body rhythms are largely determined by your T (tau), or circadian period. Basically this means, if we locked you in a room where you controlled the light, your meal-times, your entire schedule, after settling in you would see that everybody has their own natural day-length. It seems that T is strongly genetic, both in animals and humans, though it may change with age.

Turns out, most humans have a T of a little over 24 hours (24.2 I think is the most commonly cited number). So your average person, if allowed to free-run as I described, would settle into a schedule in which they started a new day every 24.2 hours. However, pretty much nobody free-runs. Your circadian rhythms are entrained mostly by light (though definitely by other sources to an extent, but light is the main one), so each day when your first exposed to light, your clock "resets," so to speak. Since most people have a T of OVER 24 hours, this means they start their day a little before they meant to. It also means they end their day a little later than they should. This is what’s known as "eveningness," or being a "night owl." Some people of course have a T of less than 24 hours, so they will start their day a little earlier than they should: Their body says yesterday is over maybe 20 minutes before the next day actually starts, and they tend to wake up early. This is termed "morning-ness" or being a "morning lark." Going along with the genetic kick, there are inherited diseases at both of these extremes: Familial Advanced Sleep Phase Syndrome (T much shorter than 24 hours) and Delayed Sleep Phase Disorder (T much longer than 24 hours). (General source for this, in a pinch.)

via Is there something genetically or physiologically that makes someone a "morning person" vs not? : askscience.