From the ancient Greeks to Medieval sleepers, a report examines nine historical facts about sleep.

Sleep is an essential part of human health; after a mere 24 hours of sleep deprivation the body starts to go haywire, and 11 days of it seems to be the record a human has managed without dying. It’s one of the human constants, but it’s also, if you think about it, a very peculiar practice. Why do we need to spend so much of our lives essentially unconscious with bizarre images playing around in our minds? And what have people throughout history thought about sleep and its role in life, psychology, and conversations with the gods? (Spoiler: quite a lot of them emphasized that last one.)

We’ve only discovered the concept of the body clock (natural shifts wakefulness and sleepiness) in the past few centuries, with its attendant procedures that cause drowsiness at night: melatonin release, increasing levels of adenosine, and so on. And we’re beginning to understand what happens to the body during sleep, including memory solidification, tissue regrowth, and growth hormone release. But sleep’s had a slightly peculiar ride throughout history, from being targeted as a potential occasion for demon possession to being utilized for (thankfully useless) brainwashing techniques. Sleep science is still expanding, but it’s been an interesting road.

Here are nine of the most fascinating episodes in the history of sleep, from hydraulic spirits in the skull to alarm clock shock-cages. If you find yourself dreaming of brain-floods and ancient Greek oracles tonight, it is officially my fault.

1. We Used To Separate Sleep Into 2 Parts Of The Night

The idea of the solid eight hours of sleep (or nine, or whatever science is currently recommending) is actually a modern creation, necessitated by the Industrial Revolution. Before that point, medieval society actually had two sleeps a night, the first and second sleeps, with a gap of wakefulness in between to eat, pray, talk, make love, or entertain yourself. Evidence suggests that the idea of segmented sleep is actually ancient: Professor Roger Ekirch, from the Department of History at Virginia Tech, who brought the idea of segmented sleep back into the mainstream in 2013, found references to it as far back as Homer’s Odyssey.

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