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. 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. 
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.
- Medical College of Wisconsin, Sleep: A Dynamic Activity
- National Institute of Neurological Disorders and Stroke, Brain Basics: Understanding Sleep
- Basics of Sleep Behavior: NREM and REM Sleep
- A Case of the Jerks by Kaitlyn Syring, University Daily Kansan, February 28, 2008
- Fisch, Bruce J. Epilepsy and Intensive Care Monitoring: Principles and Practice. New York: Demos Medical, 2010.
- Askenasy, J. J. M. (2003). "Sleep Disturbances in Parkinsonism". Journal of Neural Transmission (Springer-Verlag): 125–50.