- Blame it on the Industrial Revolution. That's when artificial light was invented, totally screwing up an innate sleep cycle that had us snoozing in a completely different way than we do today.
Before the electric light bulb, people slept in shifts....
"If somebody came to a physician 200 to 300 years ago and said they slept all night, that would be abnormal," says Dr. Robert Israel, medical director of Unity Health System's Sleep Disorders Center in Greece and Brighton.
Everybody, he explains, "would go to sleep when it got dark out, sleep three to four hours, wake up spontaneously with energy to do chores, then sleep another three to four hours."
It was a remarkably productive period of my life. I was teaching forty hours a week at a university, fifteen of it overtime, in the mornings and evenings. During the five-hour break between classes, I'd go home for time with the wife and kids. I'd go to bed early, wake up around 2:00 AM, work on my soon-to-be-published re-tellings of Fifty Famous Stories, The Adventures of Tom Sawyer, The Adventures of Huckleberry Finn, and The Autobiography of Benjamin Franklin, and sleep a couple of more hours around 4:30 or 5:00 AM.
Mark's Daily Apple, source for information about "primal living in the modern world," has a good article on the topic — What is Biphasic Sleep?