Here’s Why Your Body Jerks While Falling Asleep


5 min read
Here's Why Your Body Jerks While Falling Asleep

The human body can do some strange things. The transition from awake to asleep is a prime time for the body to put its strangeness on display. Some people will experience some twitching or jerking during this process. Perhaps it’s you, or your partner. What is going on here? Let’s take a look.

What Are They?

What Are They

As we begin the transition to sleep, sudden twitches escape our brains, causing our arms and legs to jerk. These twitches are known as hypnic jerks.

Some people are startled by them, others are embarrassed. We do know they are not all that uncommon. As many as 60 to 70 percent of people have reported experiencing hypnic jerks, per the International Classification of Sleep Disorders, a manual used to diagnose sleep-related issues.

Also known as a hypnagogic jerk or sleep start, a hypnic jerk is basically an involuntary movement, usually of a large muscle group, that occurs as you transition from wakefulness to sleep, according to Rafael Pelayo, a sleep specialist at the Stanford Sleep Medicine Center. Nobody knows exactly what causes them, but they could represent the side effects of a hidden battle for control in the brain that happens each night between wakefulness and dreams.

Normally we are paralyzed while we sleep. Even during the most vivid dreams, our muscles stay relaxed and still, showing little sign of our internal excitement. Events in the outside world usually get ignored. Experiments have shown that even if you sleep with your eyes taped open and someone flashes a light at you it is unlikely that it will affect your dreams.

But the door between the dreamer and the outside world is not completely closed. Two kinds of movements escape the dreaming brain, and they each have a different story to tell.

Brain Battle

Brain Battle

The most common movements we make while asleep are rapid eye-movements. When we dream, our eyes move according to the focus of our dreams. If, for example, we dream we are watching a game of tennis, our eyes will move from left to right with each volley. These movements generated in the dream world escape from normal sleep paralysis and leak into the real world. Seeing sleeping persons’ eyes move is the strongest sign that they are dreaming.

Hypnic jerks aren’t like this. They are most common in children when our dreams are most simple and they do not reflect what is happening in the dream world. If you dream of riding a bike you do not move your legs in circles. Instead, hypnic jerks seem to be a sign that the motor system can still exert some control over the body as sleep paralysis begins to take over.

Rather than having a single “sleep-wake” switch in the brain for controlling our sleep (i.e. ON at night, OFF during the day), we have two opposing systems balanced against each other that go through a daily dance, where each has to wrest control from the other.

Deep in the brain, below the cortex (the most evolved part of the human brain) lies one of them: a network of nerve cells called the reticular activating system. This is nestled among the parts of the brain that govern basic physiological processes, such as breathing. When the reticular activating system is in full force we feel alert and restless – that is, we are awake.

Opposing this system is the ventrolateral preoptic nucleus. ‘Ventrolateral’ means it is on the underside and towards the edge in the brain. ‘Preoptic’ means it is just before the point where the nerves from the eyes cross. We call it the VLPO. The VLPO drives sleepiness, and its location near the optic nerve is presumably so that it can collect information about the beginning and end of daylight hours, and so influence our sleep cycles.

As the mind gives in to its normal task of interpreting the external world and starts to generate its own entertainment, the struggle between the reticular activating system and VLPO tilts in favor of the latter. Sleep paralysis begins.

What happens next is not fully clear, but it seems that part of the story is that the struggle for control of the motor system is not quite over yet. Few battles are won completely in a single moment. As sleep paralysis sets in remaining daytime energy kindles and bursts out in seemingly random movements. In other words, hypnic jerks are the last gasps of normal daytime motor control.

Some people report that hypnic jerks happen as they dream they are falling or tripping. This is an example of the rare phenomenon known as dream incorporation, where something external, such as an alarm clock, is built into your dreams.

When this does happen, it illustrates our mind’s amazing capacity to generate plausible stories. In dreams, the planning and foresight areas of the brain are suppressed, allowing the mind to react creatively to wherever it wanders, much like a jazz improviser responds to fellow musicians to inspire what they play.

As hypnic jerks escape during the struggle between wake and sleep, the mind is undergoing its own transition. In the waking world, we must make sense of external events. In dreams the mind tries to make sense of its own activity, resulting in dreams.

While a veil is drawn over most of the external world as we fall asleep, hypnic jerks are obviously close enough to home – being movements of our own bodies – to attract the attention of sleeping consciousness. Along with the hallucinated night-time world, they get incorporated into our dreams.

What Should You Do?

What Should You Do

A hypnic jerk is, “a clear signal from the body, telling you it wants to sleep,” Pelayo says. “Nothing else is going to satisfy that urge.”

If you’re experiencing hypnic jerks, but sleep isn’t an option – like during a class lecture or a meeting at work – focus on something about that scenario that’s more important than dozing off, Pelayo suggests. Make eye contact with the speaker. If you anticipate snoozing during the presentation, drink coffee beforehand, since it’ll take 15 or 20 minutes to take effect. If it’s nighttime, close Netflix, turn off the light, and go to bed.

And if you want to prevent hypnic jerks, make sure you clock in the recommended 7 to 9 hours of sleep per night.

So, there is a pleasing symmetry between the two kinds of movements we make when asleep. Rapid eye movements are the traces of dreams that can be seen in the waking world. Hypnic jerks seem to be the traces of waking life that intrude on the dream world.

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