It happens to everyone. You’re about to fall asleep when suddenly you jerk awake thinking you just fell off a cliff. This experience is called hypnic jerk or sleep start. Carl Bazil, M.D. Ph.D., director of the Sleep Disorders Center at New York–Presbyterian/Columbia University Medical Center tells The Cut, “It’s a sudden increase in muscle activity that happens to just about everybody and can be quite literally startling, though the intensity depends on the person.”
While experts have yet to figure out what exactly causes sleep start, Dr. Brazil believes that there’s a struggle between your brain system that tells you to fall asleep and the one that tells you to stay awake. Sometimes the “staying awake” system puts up a fight and sleep start happens. It’s nothing to worry about. Dr. Brazil compares this phenomenon to having the hiccups, which actually makes a lot of sense. So, there you have it!