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You're drifting off to sleep, when suddenly you feel like you're plunging off a cliff -- and you jerk awake. The jolt is disorienting, and you must try again to fall asleep.

As many as 70 percent of people experience sleep starts or hypnic jerks while falling asleep, says Dr. William Kohler, medical director of the Florida Sleep Institute and director of the pediatric sleep services at Florida Hospital, Tampa.

“A hypnic jerk or sleep starts are a perfectly normal occurrence that is almost universal,” explains James K. Walsh, executive director and senior scientist at St. Luke’s Sleep Medicine and Research Center in St. Louis.

“It involves a total body experience where your muscle contracts therefore your limbs jerk or your body twitches. They generally occur during the transition between wakefulness and sleep. All of these things are very, very brief, lasting a half second or less.”

Hypnic jerks are myoclonus twitches, or involuntary muscle spasms, but sleep starts occur during hypnagogia, the stage when the body is falling asleep.

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While most people have felt hypnic jerks, a small number of people experience the frightfully-named exploding head syndrome, the sensation that there is an explosion, crashing cymbals, or thunder near (or in) one’s head. Exploding head syndrome is so rare that it is mostly reported by individual case studies. While exploding head syndrome distresses people with it, both Walsh and Kohler stress that this, too, is normal and not a sign of any problem, physical or mental.

“They’re healthy people with a very unpleasant experience,” explains Walsh.

Movement plays a role in sleep — involuntary twitches commonly take place during rapid eye movement (REM) sleep, but these jolts occur with dreams whereas hypnic jerks occur before the body can dream.

“Some people think [hypnic jerks] might be associated with anxiety and stress or with unusual or irregular sleep schedules. The exact nature of why it occurs is not really clear,” says Kohler.

While the cause remains unknown and little research is done on hypnic jerks (they are considered harmless and normal and are often too fleeting for observation), sleep doctors and researchers theorize about why they occur.

Walsh says that he, like others in the field, speculate that as the body falls asleep it goes through mini-REM-type periods where the muscles slacken and dreamlike feelings might start.

Brainwaves occurring during hypnagogia resemble brainwaves during REM sleep, which could explain the physiological changes that occur when falling asleep. During REM our heart rate, breathing, and nervous system act erratically and if the body experiences flashes of REM while entering sleep, these irregularities could contribute to twitches. Most assume the hypnic jerks occur because the body begins relaxing.

While the visceral sensation of tumbling out of bed or plummeting off a cliff feels scary as it occurs, most people do not experience sleep starts frequently enough to seek medical treatment. Kohler says if hypnic jerks inhibit sleep, a person should consult a sleep medicine doctor. He adds that a better sleep routine -- such as having a resting period prior to bed; avoiding food, smoking, and caffeine; and going to bed and waking at the same time -- improves overall sleep.

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