A growing body of evidence, garnered from both science and history, is beginning to suggest that the eight-hour sleep cycle may not be most natural arrangement for humans after all. One experiment conducted in the 1990s, for example, seemed to indicate that when completely left to their own devices, people would sleep for four hours, then wake for one or two hours before falling into a second four-hour sleep.
More recently, historians have uncovered a wealth of historical evidence that humans used to sleep in two distinct segments, including diaries, court records, medical books and literature. The historically recent change to this pattern could be the root of a condition called sleep maintenance insomnia, where people wake during the night and have trouble getting back to sleep
According to BBC News:
“... [R]eferences to the first and second sleep started to disappear during the late 17th Century. This started among the urban upper classes in northern Europe and over the course of the next 200 years filtered down to the rest of Western society ... In 1667, Paris became the first city in the world to light its streets ... [B]y the end of the century, more than 50 of Europe's major towns and cities were lit at night. Night became fashionable and spending hours lying in bed was considered a waste of time. “