Have you ever been lying in bed, trying to get to sleep, and suddenly felt like you were falling? If you have, then you’ve experienced a hypnic jerk.
A hypnic jerk is a positive myoclonic twitch that occurs during hypnagogia, the state between being awake and being asleep. Myoclonus is a sudden, involuntary muscle twitch and a positive myoclonic twitch is one that causes a muscle, or a group of muscles, to suddenly contract.* A hiccup is a myoclonic twitch affecting the diaphragm.
The exact cause of hypnic jerks is unknown, though I have heard it suggested that it is linked to human beings’ origins as tree-dwelling primates, or that it is a defence mechanism designed to jerk you back into consciousness if the body thinks it is “shutting down” too quickly as you fall asleep.
* A negative myoclonic twitch causes a muscle or muscle group to relax.