Have you ever experienced a sudden jerk of your body when you start to doze off? This phenomenon is called "hypnic jerk," which occurs when your brain relaxes and your muscles loosen. It feels like you are falling. This is more common when you are tired or feeling stressed. It is also said that if you suddenly fall asleep while concentrating on something, you are more likely to experience this phenomenon. This may happen because your brain switches to relaxation mode too quickly.