The Secret World of Sleep: 4 Weird Things About Catching Zs

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What happens after we head to bed each night? Do we simply snooze, or do we enter into a dimly lit place of wonders, terrors and peculiar science? Given that this is mental_floss, do you even have to ask?

1. Fatal familial insomnia

Have you ever not been able to fall asleep, yet felt so tired you think you might just die? Those unfortunate enough to have this disease actually do. An exceptionally rare, heritable condition, it's 100 percent fatal. According to scientists who have studied the disease, it’s caused by malfunctioning brain proteins. The brains of those afflicted look much like sufferers of Creutzfeldt-Jakob disease (the human form of mad cow disease).

But don’t freak out just yet—you almost certainly don’t have fatal familial insomnia. It’s been documented in only 28 families worldwide, and only five of those are in the United States.

2. First and second sleep

The usual eight hours a night aren't the only way to snooze. Historians have found that before the industrial age, people actually slept in two parts. The first sleep, right after sundown, was for a few hours. Around midnight or 1 a.m., folks would get up, have a bite to eat, converse, possibly make love, and eventually settle down for their second sleep.

Virginia Tech historian Roger Ekirch theorizes that the two-part system is actually more natural for humans, and that many sleep problems stem from our insistence that we stay in bed for eight hours straight.

“For most of evolution we slept a certain way,” sleep psychologist Gregg Jacobs told the BBC. “Waking up during the night is part of normal human physiology.” So if you're wide awake tonight, don't beat yourself up. Take the opportunity to live like your ancestors.

3. Hypnic jerk

You’re drifting off to sleep. It’s peaceful, your white noise machine is on, and all is well with the world. Until it’s not. You’re jolted back to awareness with a peculiar muscle spasm. What just happened?

You just experienced a hypnic jerk—and you’re not alone. Researchers believe they happen to some 70 percent of the population. What causes the condition isn’t clear, but it can be worsened by anxiety, over-exertion before sleep, or caffeine.

And while most people have experienced a hypnic jerk, just be glad you likely haven’t had to deal with exploding head syndrome

4. Demon on your chest?

Some 40 percent of all people have experienced at least one episode of sleep paralysis—disturbed REM sleep in which you feel trapped and unable to move. While the experience can be terrifying, it isn’t actually dangerous, according to sleep experts.

Multiple cultures have explained the phenomenon by claiming that demons are pinning down the sleeper (in medieval legends, we know those demons as a succubus or incubus). The human brain wants to explain the unexplainable—and if it has to blame evil supernatural forces, so be it.