Hypnic Jerk

A hypnic jerk is a brief, sudden, involuntary contraction of the body's muscles that occurs as a person is beginning to fall asleep, often causing them to jump and wake suddenly for a moment; it is one form of the involuntary muscle twitches known as myoclonus. Physically it resembles the jump a person makes when startled, sometimes with a sensation of falling, and it can come with a rapid heartbeat, quickened breathing, sweating, a peculiar feeling of shock or of falling into the void, and sometimes a vivid accompanying dream image or hallucination; it is reported more often in people with irregular sleep schedules. Hypnic jerks are common physiological events: around 70 percent of people experience one at least once in their lives, and about 10 percent experience them daily, and they are mostly benign, though when they are especially frequent or severe they have been reported as a cause of sleep-onset insomnia.