This body movement is called hypnic (or hypnagogic) or myoclonic jerk by doctors and scientists. This is also known as “sleep onset” and it can cause you to have insomnia.
This type of sensation is normal and can occur before people enter the deep stages of sleep. Doctors and scientists aren’t sure why our bodies do this, but they have a few theories.
One theory is that the brain misunderstands what happens when our muscles relax before sleep. Of course, muscle weakness is normal, but the brain is confused. For a minute I think it’s falling on you. In response, the brain tightens your muscles, a way to “catch yourself” before you fall, which shocks your body.
Moving these bodies will wake you up, but nothing to worry about. Many people go straight to sleep afterwards.