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. You may feel as if you are falling. It often happens 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 can be considered a malfunction of the brain when it suddenly switches to relaxation mode.