If you ever twitch while falling asleep, you’re not alone. In fact 70% of the population experience this phenomenon, called a hypnic or hypnagogic jerk, is so named because it occurs mostly during the hypnogogic state, or the period of transition from being awake to being asleep. There are a few theories for why hypnic jerks occur. First, some scientists believe it’s a by-product of evolution, where our bodies associate muscle relaxing with the potential to fall out of a tree, and jerk them to ensure we are still secure. On the other hand, there’s some evidence that stress, caffeine, anxiety or exercise can cause these muscle jerks. Perhaps most interesting is the theory that hypnic jerks are the result of our bodies testing to see if sleep paralysis has taken effect. During REM sleep the body is paralyzed in order to prevent a dreamer from acting out their dreams. Some scientists believe hypnic jerks occur when the brain sends electrical signals to muscles to test for this paralysis, but it has not completely set in. Whatever the reason, hypnic jerks are completely normal and safe, unless of course, you’re kicking your bedmate.