A hypnic jerk is an involuntary spasm or twitch of the muscles that literally jerks a person awake. It usually occurs in the lightest stages of sleep, often just as someone is falling asleep, but not always. The startling spasm also is called a hypnagogic massive jerk, a myoclonic jerk, or a sleep start. People often describe it as a falling sensation or an electric shock, and it is a common, completely normal experience. Repeated movements that wake a person up during the night are usually caused by a different condition, like restless leg syndrome or periodic movement disorder.
Causes and Risk Factors
This muscle jerk, technically known as a myoclonus, most commonly occurs when a person is sleeping in an uncomfortable position or is overtired. 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 the muscles relaxing. The body undergoes changes in temperature and breathing as well as a person drifts off to sleep, and the brain could misinterpret this as a sign of falling. Another theory suggests the body reacts to falling asleep much in the way that a person may twitch when dying, and the hypnic jerk is a reflex used to keep the body functioning.
When a person hasn't had enough sleep or is trying to not fall asleep, the involuntary spasms may happen more often. Sleep experts theorize that sleep deprivation, stress, and fatigue can contribute to the frequency of these movements. Caffeine may also be a factor, because it makes it harder for a person to relax. Some people also report that drinking alcohol makes sleep starts more frequent.
To make these spasms less likely, health professionals recommend making sure the bedroom is comfortable and relaxing. People should avoid drinking caffeine or alcohol before bed, and avoid strenuous activities like exercise. If a mattress is old or lumpy, a new one may make a difference. A "white noise" machine can be used to block out annoying or loud sounds that may make falling asleep more difficult.
Auditory and Visual Sleep Starts
A hypnic jerk can also occur as a person wakes up, but this is rare. In some cases, the spasm can affect the auditory nerves as well as the muscles, in a phenomenon known as an auditory sleep start. When this happens, the person hears a very loud snap or cracking sound that seems to come from the center of the head as he or she wakes up. Some people have also experienced visual sleep starts, in which a blinding flash of light awakens the sleeper, but these cases are extremely rare.
Other Reasons for Movement During Sleep
While some slight twitching and body adjustment is common during sleep, some people experience uncontrolled movements in the rapid eye movement (REM) phases, which is when most dreaming occurs. During these periods of deep sleep, all voluntary muscle activity usually stops; in rare cases, however, this temporary paralysis is disrupted, and people may move or act out their dreams. This is not a hypnic jerk, although abrupt and violent movements might wake the sleeper.
Another condition, called periodic limb movement disorder (PLMD), causes a sleeper to experience muscle twitches frequently throughout the night, sometimes as often as every 30 seconds or less. These movements usually occur during the lightest stages of sleep, as the body is relaxing, and often wake the sleeper or lead to sleep that's not very restful. The causes of PLMD are not known, and it's not clear if this condition is related to hypnic jerking.
Several other conditions can also cause a person to wake up suddenly during the night. Restless leg syndrome (RLS), in which a person experiences discomfort or pain in the legs and has the urge 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.