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.
The NINDS explains that myoclonic twitches or jerks cannot be controlled and sometimes happen when a person tries to make a movement or in response to an external event. The simplest form of myoclonus involves a muscle twitch followed by relaxation. Jerks or “sleep starts” while falling asleep is a normal occurrence in healthy persons and does not cause difficulties. However, widespread myoclonus includes persistent, shock-like contractions in a muscle group. In some cases, it starts in a particular area of the body and spreads to muscles in other regions.
Severe cases cause distortion in movement and significantly limit a person’s ability to eat, talk or walk, says the NINDS. These types of myoclonus indicate an underlying disorder in the nerves or brain. Possible causes include head or spinal cord injury, infection, brain tumor, kidney failure, liver failure, and chemical or drug poisoning.