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Why Do Humans Sleep in 8-Hour Cycles?

December 21, 2017 in History

By Natalia Mehlman Petrzela

'Girl with a Candle', late 17th century painting in the collection of the State A Pushkin Museum of Fine Arts, Moscow. (Credit: Fine Art Images/Heritage Images/Getty Images)

Get your eight hours. A command so familiar it feels timeless: humans just need their eight hours of sleep. It’s natural. But our past sleep schedules challenge the idea that anything, even our biological needs, truly transcends historical boundaries.

For much of recorded history, humans have actually slept eight hours, but in two distinct phases of approximately four hours each. As scholar Roger Ekirch uncovered through his exhaustive historical study of literature, art, and diaries, people would once head to bed when it got dark, sleep for four hours, wake for a while, and slide into a “second sleep” for another four hours. People didn’t just toss and turn along in between their two sleep sessions: they would contemplate their dreams, read by candlelight, or have sex.

Writers from Livy to Plutarch to Virgil to Homer all referred to this structure, as well as medieval Christian and African tribal cultures. But the “biphasic sleep” pattern, which was governed by the natural timing of nightfall and sunrise, didn’t last in the modern era.

As artificially illuminating the night sky became more affordable, life—and soon sleep habits—changed. Fifty European cities introduced tax-supported street lighting by 1700 and made it safe and socially acceptable to move about publicly after dark, a time of day that had previously been considered the domain solely of prostitutes and other suspicious characters. The assumption that nothing good could go on at night was so widespread that until the arrival of artificial lighting, citizens often freely emptied their “piss-pots” out of windows after dark.

In the United States, Baltimore became the first city to be lit by gas in 1816; a century later, electricity in streets and in growing numbers of homes meant nightfall no longer ensured the inescapable darkness that had dictated beginning one’s first sleep soon after. Going out at night became a fashionable social pastime, pushing bedtime later and bringing the two separate sleep phases closer to the single stretch we know today.

‘Girl with a Candle’, late 17th century painting in the collection of the State A Pushkin Museum of Fine Arts, Moscow. (Credit: Fine Art Images/Heritage Images/Getty Images)

It was industrialization that solidified the single sleep as a social norm. Especially in the cities that increasingly revolved around factory production, a newly formalized workday structured daily life and a fascination with productivity meant that spending hours lolling around in the middle of the night was considered slothful.

School schedules also became …read more


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