Kicking your partner in your sleep? Don't be a Hypnic Jerk!

Have you ever been peacefully falling asleep when you are suddenly interrupted by the terrifying, virtually subconscious sensation of free-falling? You react violently, unconsciously jerking your body into action and kicking anyone happens to be around you, only to realize that you are safe in your bed. This potentially embarrassing automatic reaction is actually an extremely common but largely unknown sleep phenomena called a hypnic jerk.

A hypnic jerk, or sleep start as it is otherwise called, is an involuntary muscle movement made during sleep, or while coming into or out of sleep. They often occur in the first stage of sleep, the transition between a wakeful and sleeping state. These movements are generally only noticed when they are forceful enough to wake someone up. Many people may experience hypnic jerks but have no idea because not all movements are strong enough to bring a person back to a wakeful state.

Most people also experience very slight jerks during REM sleep, when the body has lost virtually all muscle tone (thanks to REM atonia, the paralysis that kicks in at this time of night) but may be attempting to act out the activity of a dream.

Hypnic jerks that take place while a person is waking up are less frequent and are known as auditory sleep starts because they are frequently accompanied by a perceived shattering or cracking noise.

Hypnic jerks are relatively common and are typically no cause for concern or embarrassment. They are estimated to affect roughly 10% of the population every night, but up to 80% of people occasionally experience them (think of anytime you have seen a student in class or in the library trying to fight off sleep and suddenly jerk into a wakeful state).

In almost all cases hypnic jerks pose no immediate danger or health concern, except for the fact that they can disrupt the sleep of a bedmate. Besides being annoying, if this happens frequently the bedmate's alertness in the following days could be compromised, which can pose serious risks.

What Causes A Hypnic Jerk?

Sleep scientists in general have not paid much attention to sleep starts because they usually pose such a minor concern. As such, little is known about the biological mechanisms that cause a hypnic jerk. Some scientists though have theorized that they are the brain's natural (but slightly confused) response to the normal bodily changes as we move into sleep: temperature dropping, breathing slowing, and muscles relaxing.

Many people experience the sensation of falling before a hypnic jerk occurs. Some researchers have attributed this falling sensation to the muscular relaxation that takes place as we "fall" asleep, a similar relaxation to that which we would experience in a freefall. The brain reacts to this by kicking our body into action, giving a person a rapid return to awareness and restoration of muscular tone.

A more morbid explanation for the violent jerking is that the movements are our bodies' attempt to keep us alive. The movements observed in a person hypnic jerking look similar to those in a body twitching while it is dying. It has been said that sometimes the brain, especially when fatigued or fighting off sleep, may even confuse sleep with death, activating some of our body's emergency "stay alive" mechanisms. This could be why people often wake from hypnic jerks in a state of panic or anxiety.

Studies have shown that experiencing sleep starts is more common for people who are sleep deprived, people with irregular sleep schedules, and people involuntarily falling asleep. Eliminating your sleep debt by sleeping more and establishing a regular sleep schedule should reduce the frequency and intensity of hypnic jerk episodes (and increase your overall quality of life!).

Hypnic jerks can also result from sleep anxiety and difficulty falling asleep due to discomfort, so it is important to sleep safely and comfortably.

When You Should Be Concerned

If your partner has noticed that you jerk very regularly or for long periods of time, it is likely that you are not experiencing just simple sleep starts. This significantly more rare and slightly more serious condition is known as periodic limb movement disorder.

Unlike hypnic jerks, periodic limb movement disorder is characterized by leg movement at regular intervals of 20-40 seconds for long periods of the night (even multiple hours). The sufferer often sleeps through these leg movements, but they can disrupt sleep and are often even more disruptive for a partner. Little is known about the direct cause of periodic limb movement disorder, but it has been noticed more frequently in patients with other sleep disorders or nervous system damage.

Another more serious version of the hypnic jerk is restless leg syndrome. This involves leg movement similar to that found in periodic limb movement disorder but not as regular. It is also accompanied by uncomfortable sensations in the legs and can severely disrupt healthy sleep.

You should contact your doctor if you experience frequent, repeated, or severely disruptive hypnic jerks, as they may actually be symptoms of periodic limb movement disorder or restless leg syndrome. Both of these conditions have a wide variety of causes and could be the symptom of even a greater problem.