Explanation: This is called scientifically “Hypnic Jerk” or “Myoclonic Jerk” – simply put, it is a sudden muscle contraction during sleep, especially common in the leg muscles. When sleeping, breathing and heart rate slow down, body temperature also drops, and muscles are completely relaxed to rest and recover. This causes the brain to misunderstand, thinking that the body is in a state of “real fall” and quickly sends a signal to contract the muscles to help the body regain balance, causing you to wake up with a start.