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It's been a long day at work and you want nothing more than a little shut-eye. You groggily walk up the stairs to your bedroom shedding layers of clothing as you go. Your body flops down on the bed and you shut your eyes. You start to fall asleep, the world blurs around you, your breathing slows, and you are about to drift into dreamland when suddenly your entire body convulses. You jump right off your mattress and land with a thud. What just happened? What caused you to practically jump out of your skin?

You have no idea, and the worst part is… now you’re wide awake. The phenomenon you just experienced is called hypnic jerk or sleep starts. Scientists aren’t entirely sure why the body does this, but it is a common occurrence. Between 60-70 percent of people report having experienced hypnic jerks during their life. The intensity of the experience depends on the person. Let’s look at what is actually happening in the brain and body when you jump awake out of peaceful sleep. The human brain has two main systems that control your day-to-day life.

One system is called the reticular activating system, which sits just below the cortex. It is located near the parts of the brain that controls basic functions such as breathing and your heartbeat. When the reticular activating system is turned on you feel alert. In other words, you are awake. The other system operating in the brain is the ventrolateral preoptic nucleus, which is located on the underside of the brain, right near where the nerves for your eyes cross. Fun fact, the left side of your brain controls your right eye and the right side of your brain controls your left eye. Anatomy is cool.

The ventrolateral preoptic nucleus or VLPO is what causes the body to sleep. It is hypothesized that the VLPO is located near the visual cortex so that it can collect information about the time of day based on the intensity of light entering the eyes. Once you start to drift off to sleep the brain starts to rely more heavily on the ventrolateral preoptic nucleus and less on the reticular activating system. This does not work like an on-off switch, but instead, it’s like a slider that slowly shifts control from one region of the brain to the other.

The transition period is where things can get interesting. The term hypnic jerk got its name from the jumping or twitching sensation most often associated with the hypnogogic stage of sleep. This is the transition stage when your body goes from being awake to being asleep. A hypnic jerk most likely occurs when the awake part of your brain has not completely shut down before the sleep part of your brain tries to take over.

Think of it as a battle within yourself. Who will win? The sleepiness or the awareness? When you sleep your body falls into what is called sleep paralysis. This ensures that when you dream of running a marathon your legs don’t kick your covers off, or the person next to you. It is also why you can jump, swim, ride a bike, and do all sorts of other activities in your dreams, and when you wake up you are still comfortably lying in your bed. However, hypnic jerks seem to happen when the motor system of the brain, which controls muscle movements, is still fighting for control even if you’re trying to sleep.

People experience the battle from being awake to falling asleep in different ways. Many people experience hypnic jerks often, but just don’t wake up. Instead, their sleeping partner might complain upon waking up that they were kicked by the hypnic jerker. But the person who had the hypnic jerk has no recollection of kicking their sleeping companion at all. These jerks or twitches are involuntary, but can still hurt the person sleeping next to you. The involuntary muscle movements are called myoclonus. It's actually the same process that causes hiccups.

Again, anatomy is cool! But you’re not necessarily concerned about the weak hypnic jerks that just cause a twitch. No, you probably want to learn more about why you sometimes jump straight into the air when a hypnic jerk occurs. The reason is that each hypnic jerk may vary in strength and from person to person. Some spasms can be strong enough to cause you to jump awake, while others are just little tremors you sleep straight through. Either way, hypnic jerks are the result of your brain battling itself for control between the real world and the dream world.

That might not be the most satisfying answer, but it is the best one scientists have at the moment. Scientists have several theories as to why hypnic jerks occur. One of the more interesting ones is that they are byproducts of human evolution. Our ancient ancestors lived in trees, and only recently came down from those trees to spend most of the time on the ground. Recently in evolutionary time that is. Our DNA may still be programmed with warnings, such as hypnic jerks, that are left over from our distant ancestors.

For example, living in trees is great for protection, but when you fall asleep it can be very danger rous. A thirty-foot fall from a tree will kill you just as quickly as a predator on the ground. Hypnic jerks may be a leftover warning system to ensure that the body is secure when sleeping in the canopy of a forest. Theories about the current human condition may shed light on hypnic jerks as well.  There is evidence that stress, caffeine, anxiety, and exercise may cause more frequent hypnic jerks.

Our lifestyle may play a large role in the likelihood and frequency of sleep starts. One of the more interesting theories is that hypnic jerks are actually our bodies testing to see if sleep paralysis has set in.

Imagine for a moment that you are your own brain, I know, trippy, right? You are slowly shutting down for the night so that you can enjoy some relaxing dreams and recharge your batteries. But as your brain, you want to ensure that your body won't roll out of bed and crash to the floor. Or when you start dreaming about being in a rock n’ roll band you won't start jamming out on the air guitar in your bed.

So just to make sure the body is in sleep paralysis you shoot signals through the muscles to check and see if they move. You push a little too hard and whoops! You hypnic jerk your body awake and you’re back to square one. But at least you didn’t play a whole rock concert out in your bed. A less common, but just as interesting trick of the brain is dream incorporation. Sometimes people claim that they were falling or kicking something in their dreams and they woke up jerking or kicking in their bed.

This phenomenon shows the brain’s ability to create detailed stories in your dreams. Some are so real, and so close to a certain sensation, your body actually plays out the action from the dream itself. But should you be worried about hypnic jerks and how it affects your body? Are they harmful? Well, most scientists agree that they are completely normal and safe, other than maybe kicking your sleeping mate.

But there have been some accounts that hypnic jerks can be anxiety-inducing events. There are stories of sleep-deprived patients who have told doctors that they have intense hypnic jerks every night at the same time.

This can be a huge annoyance, but it is not physically or mentally harmful. Hypnic jerks can happen to people of all ages. Research shows that adults are more likely to complain about hypnic jerks than children, but that just could be because adults tend to get more cranky when their sleep is interrupted.

In adults, hypnic jerks seem to be correlated to caffeine intake, stress, and exercise right before falling asleep. If you’re tired of jumping awake at night there are some recommended ways to stop hypnic jerks from occurring.

With all things we do, humans often crave routines. Like having a cup of coffee in the morning or going to the gym at 6:43 every evening. Sleep is no different. The body, and mind, might relax more if you routinely go to bed and wake up at the same time every day. This could lessen the likelihood of hypnic jerks. You can also reduce the effects and chances of hypnic jerks by avoiding things that stimulate the brain right before trying to go to sleep.

The more relaxed you are, the easier it is for your brain to transition from being awake to being asleep. Some suggestions to reach this zen feeling are avoiding late-night exercises before bed, not drinking coffee or other caffeine-containing beverages close to sleeping time, turning down the lights prior to getting into bed, and doing relaxing meditations to prep your brain for sleep mode.

If you ever are drifting off to sleep and suddenly jump, it is only your brain battling with itself.

Hypnic jerks can range from small twitches to convulsions that cause you to jump awake. However, they are not dangerous, except perhaps to the person sleeping next to you. If you want to lessen the likelihood of punching or kicking your bedmate in your sleep, remember to stay away from exercise, caffeine, and stimulating activities before bed.

Here’s to a good night’s sleep! What were you dreaming about last time you had a hypnic jerk? Were you kicking a soccer ball, falling off a cliff, or just enjoying a relaxing dream on the beach?