It’s one of the strangest sensations you can experience when you’re trying to get into bed for the night. Just as you think you’re falling asleep, you suddenly feel like you’re in free fall. Your stomach does somersaults and your body spasms involuntarily as you lie in bed. Your limbs, or even your entire upper body, may shake or thrash during the process. There’s no logical reason why you felt like you briefly entered a fight-or-flight mode and the calm you recently achieved is erased in an instant. This experience may leave you confused, bored, or just plain weird — and to make matters worse, it’s probably not the first time this has happened. But if it’s any consolation, you’re not alone.

An estimate 70 percent of people will experience hypnic shocks at least once in their life. About 10 percent of the population may even succumb regularly to this 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 have not reached agreement on what causes this oddity often called “sleep onset,” or why it occurs.

“Hypnic jerks represent a fascinating phenomenon of sleep; however, additional studies are needed to clarify their physiology and origin,” summarize the authors of a 2018 study in the journal, Current Sleep Medicine Reports. More than six years later, this opinion is still relevant among experts in the field.

“I don’t know of any good explanation for why this happens,” says Allen Richertassociate professor of psychiatry and division director of sleep medicine in the Department of Psychiatry and Human Behavior at the University of Mississippi Medical Center. “It is an organized muscular movement, which will therefore require communications with the central nervous system. Although, to my knowledge, it is not clear where and what stimulates this movement, and why.

However, there is some knowledge about hypnic jerks. According to experts, these movements are prime examples of myoclonus or myoclonic jerks – involuntary muscle spasms similar to hiccups and epileptic seizures. But that doesn’t mean waking up suddenly shortly after turning off the lights warrants scheduling an MRI.

“We don’t see this as a pathological problem. This is a normal phenomenon,” explains Richert. “It’s certainly not a sign of a sleep disorder that we would treat, nor is it a sign of underlying issues like seizures or movement disorders.”

THE American Academy of Sleep Medicine offers a number of reasons that may be causing your particular variant of hypnagogic jerks. Unfortunately, the spectrum of explanations is so broad that it cannot really be clarified. While “high consumption of caffeine or other stimulants” and “excessive stress” could generate sleep onset, so could “previous work or strenuous exercise” and lack of sleep. Hypnagogic jerks can come from too much intensity, but they can also occur when you are exhausted.

“That’s what’s interesting,” Richert says. “A lot of times you can say, ‘Well, if you deprive yourself of sleep for a period of time, you’re more likely to experience (physical consequences).’ » But… to my knowledge, there is no evidence that sleeping more would make the situation better, or that if you don’t get enough sleep it makes it worse.

Still, many sleep researchers have proposed their own plausible theories to explain hypnic jerks. A group hypothesizes that the sensation occurs after a drop in blood pressure and relaxation of muscle tissue that causes your brain to misfire and briefly enter panic mode. While there’s no concrete evidence yet to support this claim, other experts say this feeling occurs when your brain instinctively thinks you’re likely to fall from a tree– as may have been the case for some of our primate ancestors.

(Related: Why do jump scares terrify us so much? Blame evolution.)

Richert adds that he’s also not aware of any specific medications or behavioral modifications that would increase or decrease sleep onset. For its part, the AASM recommends some of the classic pre-sleep tips: avoid caffeine and other stimulants in the hours before bed, work to reduce stress, and make sure you get enough sleep in general.

But as long as hypnic jerks — or the fear they evoke — aren’t negatively impacting your life, Richert and other experts say there’s no need to see a doctor. For now, hypnagogic jerks remain yet another mysterious human evolutionary quirk, but they’re certainly not a problem worth losing sleep over.

This story is part of the Popular Science book Ask Us Anything Serieswhere we answer your craziest and most burning questions, from the ordinary to the unusual. Do you have something you always wanted to know? Ask us.