Hypnic jerk

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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. Hypnic jerks are common physiological phenomena. Around 70% of people experience them at least once in their lives, with 10% experiencing them daily...

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