What Causes a Body to Jerk During Sleep?

James Porter/Photolibrary/Getty Images

According to the National Institute of Neurological Disorders and Stroke (NINDS), the sudden, involuntary jerking of a muscle or group of muscles while drifting off to sleep is called myoclonus. It is caused by sudden muscle contractions, also known as positive myoclonus, or muscle relaxation, which is referred to as negative myoclonus. Myoclonic jerks may occur alone or in sequence, in a pattern or without pattern.