A hypnotic jerk is an involuntary spasm or contraction of muscles that literally jerks a person. It usually occurs in the lighter stages of sleep, often just as someone is falling asleep, but not always. The startling spasm is also called a massive hypnagogic jerk, a myoclonic jerk, or a sleep onset. People often describe it as a falling sensation or an electric shock, and it's a common and completely normal experience. Repeated movements that wake a person up at night are usually caused by a different condition, such as restless legs syndrome or periodic movement disorder.
Causes and risk factors
This muscle pull, technically known as myoclonus, most commonly occurs when a person is sleeping in an awkward position or is overloaded. The American Academy of Sleep Medicine reports that up to 70% of people experience them when falling asleep. Little research has been done on the subject, but one theory suggests that it may be the result of muscle relaxation. The body undergoes changes in temperature and respiration as a person falls asleep, and the brain may misinterpret this as a sign of falling. Another theory suggests that the body reacts to falling asleep in the same way that a person might shiver when dying, and the hypnotic jerk is a reflex used to keep the body going.
When a person hasn't slept enough or is trying not to fall asleep, involuntary spasms may occur more often. Sleep experts theorize that lack of sleep, stress, and fatigue may contribute to the frequency of these movements. Caffeine can also be a factor, because it makes it harder for a person to relax. Some people also report that drinking alcohol makes them fall asleep more often.
Prevention
To make these spasms less likely, health professionals recommend making sure the room is comfortable and relaxing. People should avoid drinking caffeine or alcohol before bedtime, and avoid strenuous activities such as exercise. If a mattress is old or lumpy, a new one can make all the difference. A "white noise" machine may be used to block out distracting or loud sounds that can make it hard to sleep.
Auditory and visual onset of sleep
A hypnotic jerk can also occur when a person wakes up, but this is rare. In some cases, the spasm can affect the auditory nerves and muscles, in a phenomenon known as auditory sleep onset. When this happens, the person hears a very loud clicking or crackling sound that seems to come from the center of the head when they wake up. Some people have also experienced visual sleep onset, in which a blinding flash of light awakens the sleeper, but these cases are extremely rare.
Other reasons to move during sleep
While some slight twitching and body adjustments are common during sleep, some people experience uncontrolled movements in rapid eye movement (REM) stages, which is when most dreams occur. During these periods of deep sleep, all voluntary muscular activity generally stops; in rare cases, however, this temporary paralysis is interrupted, and people can move or act out their dreams. This is not a hypnotic jerk, although sudden and violent movements can wake the sleeper.
Another condition, called periodic limb movement disorder (PLMD), causes a sleeper to experience muscle contractions frequently during the night, sometimes as often as every 30 seconds or less. These movements usually occur during the lighter stages of sleep as the body relaxes and often wakes the sleeper up or puts them into sleep that is not very restful. The causes of PLMD are not known, and it is not clear if this condition is related to hypnotic jerks.
Various other conditions can also cause a person to wake up suddenly during the night. Restless legs syndrome (RLS), in which a person experiences discomfort or pain in the legs and has the need to move them to relieve it, is a relatively common condition. People with sleep apnea may also wake up abruptly when they stop breathing during the night.