Seriously, though, people vary in the times they like to sleep and wake—this has been the case throughout history. What’s new, however, is the recent discovery that the tendency toward being a lark or an owl is genetically determined in a similar way to the tendency to have blue or green eyes. The gene that governs sleep/wake predisposition is called Period, or PER for short. Like the gene for eye color, it comes in two different types (let’s call them PERI, which causes people to have larkish tendencies, and PERo, which causes them to have owlish tendencies). You probably know that we have two copies of all our genes, which means we have two copies of PER as well. The trick, however, is that these copies don’t have to be the same: If you have two copies of PERI, you’ll be a lark. If you have two copies of PERo, you’ll be an owl. But if you have one copy of each (which is what 50 percent of the population has), you’ll be somewhere in between. This works for eye color too, by the way: If you have two copies of EYE-COLORg, you’ll have green eyes. Two copies of EYE-COLORb you’ll have blue eyes, and a copy of each you’ll have brown eyes.
From Penelope A Lewis, The Secret World of Sleep (2013)