Are you a hypnic jerk? What that is...

Have you ever drifted off to sleep only to be jolted awake by an involuntary spasm? 

That’s called a hypnic jerk and it actually serves a purpose. Some scientists believe this automatic reaction dates back through evolution to when humans often slept in trees, which is why we may even experience the sensation of falling just prior to the spasm. That hypnic jerk would signal our tree-sleeping ancestors to reposition themselves and prevent a fall as they drifted off. 

Another theory is that the brain can sometimes confuse sleep with death, activating some of the body's emergency "stay alive" mechanisms. That helps explain why hypnic jerks often wake up in a state of panic.