From: New York Times, Rethinking Sleep by Dave Randall Sept 22, 2012, (edited)
History prof Roger Ekirch noticed references to split-shift sleep in historical literature. People would sleep for awhile then wake up for awhile and then go back to sleep again.
A character in the “Canterbury Tales” decides to go back to bed after her “firste sleep.”
A doctor in England wrote that the time between the “first sleep” and the “second sleep” was the best time for study and reflection.
A 16th-century French physician concluded that laborers were able to conceive more children because they waited until after their “first sleep” to make love.
At the same time, Tommy Wehr, a psychiatrist, was conducting an experiment in which subjects were deprived of artificial light.
At first, the subjects slept through the night. Then they began to wake up a little after midnight, lie awake for a couple of hours, and then drift back to sleep again, in the same pattern Ekirch saw in historical records and literature.
Subjects grew to enjoy the time in the middle of the night as a chance for deep thinking of all kinds, whether in the form of self-reflection, getting a jump on the next day or amorous activity.