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A hypnic jerk, hypnagogic jerk, sleep start, sleep twitch or night start, is an involuntary twitch which occurs just as a person is beginning to fall asleep, often causing them to awaken suddenly for a moment. Physically, hypnic jerks resemble the "jump" experienced by a person when startled, often 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.'" A higher occurrence is reported in people with irregular sleep schedules.
According to the American Academy of Sleep Medicine there are a wide range of potential causes, including anxiety, caffeine, stress and strenuous activities in the evening. However, most hypnic jerks occur essentially randomly in healthy people.
During an epilepsy and intensive care study, the lack of a preceding spike discharge measured on an epilepsy monitoring unit, along with the presence only at sleep onset, helped differentiate hypnic jerks from epileptic myoclonus.
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 4-7 per hour at the age ranging from 8 to 12 years old, and it decreases toward 1-2 per hour at 65 to 80 years old. 
The evolutionary explanation for the existence of the hypnic jerk remains unclear, but one possibility is that it is a vestigial reflex humans evolved when they usually slept in trees. Thus, experiencing a hypnic jerk prior to falling asleep may have been selected so that the individual would be able to readjust their sleeping position in the tree to prevent a fall from occurring.
According to Marianne Middleton, clinical coordinator at the Lawrence Memorial Hospital Sleep Disorders Center, the occurrence of hypnic jerks can become cyclical. The cycle occurs because,
If you lose sleep because you constantly jerk awake, you will become fatigued and may develop anxiety or worry about falling asleep. The more worried and tired you are, the more likely you are to jerk awake. The more you jerk awake, the more sleep you lose.