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/ Source: TODAY
By Meghan Holohan

Everyone has experienced that feeling where sleep is so close — yet suddenly the body violently jerks awake, in an almost spasm. Why does this happen? Is something wrong or is it simply an annoying tick?

“It is sort of a rough transition,” Dr. W. Christopher Winter, a sleep medicine doctor at Charlottesville Neurology and Sleep Medicine in Virginia, told TODAY. “Hypnic jerks are happening because it is sort of a transitional period in your brain from being awake and having control of your body and its movements and … transitioning to sleep.”

Hypnic jerks, also known as hypnagogic jerks or sleep starts, are extremely common. Experts believe that as many as 70% of people experience them at some point in their lives. Even cats and dogs experience them when they look like they are twitching or running in their sleep.

“The hypnic jerk is characterized by the involuntary movement of the body. It is often accompanied by a dream of falling into a void, falling off of something,” said Marissa Bowman, a doctoral student in clinical health psychology at the University of Pittsburgh. “It is more like imagery you are having, falling into sleep.”

While many people report dreams of falling, tripping, stumbling or floating through air, the brain isn’t in the rapid eye movement (REM) stage of sleep so they’re not having an actual dream. Instead it is probably an image the brain creates because it's supremely relaxed, but feels as if something’s off.

“It’s not that people are dreaming. It could be almost a reflex. For whatever reason your brain feels like it is falling because of the relaxing,” Dr. Charles Bae, associate professor of clinical medicine and neurology at University of Pennsylvania Sleep Center in Philadelphia.

While hypnic jerks are normal, they do occur more often when a person is under a lot of stress or isn’t sleeping well. Stress and poor sleep could prevent people from going through the sleep cycles properly, leading to more hypnic jerks.

“If you don’t time your sleep right, where you get to those deep sleep stages quickly, you might have the hypnic jerks and you might have to start trying to sleep all over,” Bae said. “It is the same with stress. If you are having a hard time falling asleep, you have a harder time reaching deeper sleep.”

Hypnic jerks only occur when people fall asleep. If people experience them throughout the night it might be because they are waking up and not realizing it.

“If it is happening in the middle of the night that means you are waking up,” Bae said. “That would be a time to mention it to the doctor.”

Often more worrisome conditions, such as sleep apnea, can cause waking during sleep and doctors will need to do a sleep study to understand what’s happening. But for most, hypnic jerks are perfectly normal.

“Normally it is harmless unless … it cuts into their sleep time,” Bae said.