Body & HealthFact
About 70 percent of people know that jolt before sleep.
Roughly 70 percent of people experience a hypnic jerk at least once in their life, about 10 percent daily. The cause is unresolved. One hypothesis sees it as a reflex response to the sudden muscle relaxation and falling blood pressure at sleep onset; another reads it as the brain misreading relaxation as falling.
Useful: A phenomenon can be extremely common and still scientifically unexplained.
Source: en.wikipedia.org