It’s one of the oddest sensations you can experience as you’re trying to turn in for the night. Just as you think you’re falling asleep, you suddenly feel as though you’re in a freefall. Your stomach somersaults and your body involuntarily spasms as you lay in bed. Your limbs, or even your entire upper body, may jerk or thrash in the process. There’s no logical reason why you felt as if you briefly entered a fight-or-flight mode and whatever calmness you recently attained is erased in an instant. The experience may leave you confused, annoyed, or simply weirded out—and to make matters worse, it likely isn’t the first time this has happened. But if it’s any consolation, you aren’t alone.
An estimated 70 percent of people will experience hypnic jerking at least once in their lives. Around 10 percent of the population may even regularly succumb to the sensation, also called hypnagogic jerks in reference to the hypnagogic transitional stage of falling asleep. And yet, despite its regularity and decades of documentation, researchers haven’t come to an agreement on what causes the oddity often called a “sleep start,” or why it happens.
“Hypnic jerks represent a fascinating phenomenon of sleep; however, additional studies are needed to clarify their physiology and origin,” summarizes the authors of a 2018 study in the journal, Current Sleep Medicine Reports. Over six years later, that opinion still stands among experts in the field.
“I’m not aware of any good explanation for why it happens,” says Allen Richert, associate professor of psychiatry and division director of sleep medicine in the University of Mississippi Medical Center’s Department of Psychiatry and Human Behavior. “It’s an organized muscle movement, so it’s going to require central nervous system communications. Although where and what stimulates that movement, and why, is, to my knowledge, unknown.”
There are a few knowables about hypnic…
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