Hypnic jerk

Hypnic Jerks: Overview

A hypnic jerk, also known as a sleep start, hypnagogic jerk, or predormital myoclonus, is a brief and sudden involuntary contraction of the muscles of the body, occurring when a person is beginning to fall asleep. This can often cause the person to jump and awaken suddenly for a moment. Hypnic jerks are one form of involuntary muscle twitches called myoclonus. They are common physiological phenomena, with around 70% of people experiencing them at least once in their lives and 10% experiencing them daily. Although their course is benign and they don't cause any neurological sequelae, when particularly frequent and severe, hypnic jerks have been reported as a cause of sleep-onset insomnia.

Physical Characteristics

Physically, hypnic jerks resemble the "jump" experienced by a person when startled, sometimes accompanied by a falling sensation. They are associated with rapid heartbeat, quickened breathing, sweat, and sometimes "a peculiar sensory feeling of 'shock' or 'falling into the void'". Hypnic jerks can also be accompanied by a vivid dream experience or hallucination. A higher occurrence is reported in people with irregular sleep schedules.

Causes

According to the American Academy of Sleep Medicine, there is a wide range of potential causes for hypnic jerks, including anxiety, stimulants like caffeine and nicotine, stress, and strenuous activities in the evening. They may also be facilitated by fatigue or sleep deprivation. However, most hypnic jerks occur essentially at random in healthy people. Nevertheless, repeated, intensifying twitches can cause anxiety in some individuals and disrupt their sleep onset.

Differential Diagnosis

Hypnic jerks can sometimes be mistaken for other forms of movement during sleep, such as restless leg syndrome, periodic limb movement disorder, hypnagogic foot tremor, rhythmic movement disorder, and hereditary or essential startle syndrome, including the hyperplexia syndrome. However, certain features can help to distinguish hypnic jerks from these other conditions, such as their occurrence only at sleep onset, the lack of rhythmicity or periodicity of the movements and EMG bursts, and the presence of other pertinent history.

This physiological phenomenon can also be mistaken for myoclonic seizures, but it can be distinguished by different criteria, such as the fact that hypnic jerks occur only at sleep onset, and that the EEG is normal and constant. Unlike seizures, hypnic jerks do not cause tongue bites, urinary incontinence, or postictal confusion.

Theories Behind Hypnic Jerks

Scientists do not know exactly why hypnic jerks occur, and several theories have been proposed to explain the phenomenon. One hypothesis posits that hypnic jerks are 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 suggests that the body mistakes the sense of relaxation felt when falling asleep as a sign that the body is falling, causing a jerk to wake the sleeper up so they can catch themselves.

Occurrence During Sleep

According to a study on sleep disturbances in the Journal of Neural Transmission, a hypnic jerk occurs during the non-rapid eye movement sleep cycle and is an "abrupt muscle action flexing movement, generalized or partial and asymmetric, which may cause arousal, with an illusion of falling". Hypnic jerks are more frequent in childhood, with 4 to 7 per hour in the age range of 8 to 12 years old, and they decrease to 1 or 2 per hour by 65 to 80 years old.

Treatment

To reduce hypnic jerks, it is recommended to:

• Reduce consumption of stimulants such as nicotine or caffeine
• Avoid physical exertion prior to sleep
• Consume sufficient magnesium
• Some medications can also help reduce or eliminate hypnic jerks. For example, low-dose clonazepam at bedtime may make the twitches disappear over time.

Additionally, some people may develop a fixation on these hypnic jerks, leading to increased anxiety and worrying about the disruptive experience. This increased anxiety and fatigue can raise the likelihood of experiencing these jerks, resulting in a positive feedback loop. In such cases, addressing the underlying anxiety may help reduce the frequency and intensity of the hypnic jerks.

See also

• Exploding head syndrome
• Hypnagogia
• Periodic limb movement disorder
• Rapid eye movement
• Sleep paralysis

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