A hypnic jerk is defined as a brief and sudden involuntary contraction of the muscles of the body which occurs when a person is beginning to fall asleep, often causing the person to jump and awaken suddenly for a moment. Hypnic jerks are one form of involuntary muscle twitches called myoclonus.
Physically, hypnic jerks resemble the "jump" experienced by a person when startled, sometimes accompanied by a falling sensation. Hypnic jerks are associated with a rapid heartbeat, quickened breathing, sweat, and sometimes "a peculiar sensory feeling of 'shock' or 'falling into the void. They can also be accompanied by vivid dream experiences or hallucinations. A higher occurrence is reported in people with irregular sleep schedules. When they are particularly frequent and severe, hypnic jerks have been reported as a cause of sleep-onset insomnia. Around 70% of people experience them at least once in their lives, with 10% experiencing them daily. They are mostly benign and do not cause any neurological sequelae, though in severe cases, there can be pronounced negative effects on sleep time and quality.
Causes
Risk factors
According to the American Academy of Sleep Medicine (AASM), there is a wide range of potential causes, including anxiety, stimulants like caffeine and nicotine, stress, and strenuous activities in the evening. It may also be facilitated by fatigue or sleep deprivation. Nevertheless, these repeated, intensifying twitches can cause anxiety in some individuals and a disruption to their sleep onset. Hypnic jerks are more frequent in childhood with 4 to 7 per hour in the age range from 8 to 12 years old, and they decrease to 1 or 2 per hour by 65 to 80 years old. One hypothesis posits that the hypnic jerk is a form of reflex, initiated in response to normal bodily events during the lead-up to the first stages of sleep, including a decrease in blood pressure and the relaxation of muscle tissue. Another theory postulates that the body mistakes the sense of relaxation that is felt when falling asleep as a sign that the body is physically falling. As a consequence, it causes a jerk motion to wake the sleeper up so they can catch themselves. A researcher at the University of Colorado suggested that a hypnic jerk could be "an archaic reflex to the brain's misinterpretation of muscle relaxation with the onset of sleep as a signal that a sleeping primate is falling out of a tree. The reflex may also have had selective value by having the sleeper readjust or review his or her sleeping position in a nest or on a branch in order to assure that a fall did not occur", but evidence is lacking.
Treatment
There are ways to reduce hypnic jerks, including reducing consumption of stimulants such as nicotine or caffeine, avoiding physical exertion prior to sleep.
Some medications can also help to reduce or eliminate the hypnic jerks. For example, low-dose clonazepam at bedtime may make the twitches disappear over time.
In addition, some people may develop a fixation on these hypnic jerks, leading to increased anxiety about the disruptive experience. This increased anxiety and fatigue increase the likelihood of experiencing these jerks, resulting in a positive feedback loop.