Walter A. Brown MD, Psychiatric Times:

In the course of gathering information for his book, At Day's Close: Night in Times Past, which is about night in preindustrial times, A. Roger Ekirch, professor of history at Virginia Polytechnic Institute in Roanoke, uncovered the fact that in preindustrial times before artificial illumination was widely used, persons typically slept in 2 shifts. They called the shifts "first sleep" and "second sleep." In those times, sleep was more closely associated with sunset and sunrise than it is now. Within an hour or so after sunset, persons retired to bed, slept for about 4 hours and then woke up. They remained awake for a few hours and then returned to sleep at about 2 am for another 4 hours or so. Written records from before the first century onward indicate that the period between first and second sleep afforded persons a chance for quiet contemplation, but persons also rose from bed during this interval and did household chores or visited with family and friends. Although diaries, court documents, and literature of the time indicate that this sleep pattern was widely acknowledged, this bit of history had been lost until the debut of Ekirch's work.

And:

Wehr, now Scientist Emeritus at the NIMH, conjectures that our current sleep pattern, in which we fall asleep rapidly and expect to sleep-and often do-for an uninterrupted 7 or 8 hours, may be an artifact of both chronic sleep deprivation and artificial light. When the participants in his experiments shifted from the 16-hour "days" and 8-hour "nights" customary for them (and for others in developed countries, who depend on artificial light) to the "natural winter" conditions of his experiment, they first slept for 11 hours and then began to sleep for an average of 8.9 hours, as compared with 7.2 hours under ordinary conditions. These and other data suggest that our current schedules do not allow us the sleep that we require. Wehr also observed that when given 14 hours of darkness, it took participants in bed rest about 2 hours to fall asleep compared with 15 minutes under usual conditions. He speculates that under our current conditions, we may fall asleep so quickly because we are chronically sleep-deprived.