Main points:
- In the 1990s a study was done where a group of people were sequestered into darkness 14 hours per day. After their sleep stabilized they slept for 4 hours, woke up for 1-2 hours, and slept for another 4 hours.
- In 2001 a meta study of 16 years of research suggested, historically, humans used to sleep in 2 distinct chunks. The source of information was from diaries, medical records, literature, court records, and scientific research.
- An example,
"He knew this, even in the horror with which he started from his first sleep, and threw up the window to dispel it by the presence of some object, beyond the room, which had not been, as it were, the witness of his dream." Charles Dickens, Barnaby Rudge (1840)
- During this wakened interlude people were active (smoking, visiting neighbors, reading, writing, praying, etc.) There were even special prayers for this awake phase. And a doctors manual suggested this is the best time to conceive.
- References to the two sleep periods started to disappear in the late 17th century starting with the upper class in northern Europe. By 1920 all references were gone.
- The 2001 study suggests that public and home lighting and more evening entertainment were the cause of its demise.
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