We’ve all experienced that unpleasant sensation: drifting off into a lovely snooze, only to find yourself falling off a building/cliff/sidewalk and wake up with a jolt. But why does this happen?
The weird feeling is known as the “hypnic jerk” and is often accompanied by a visual hallucination, (or, you know, a dream).
Hypnic jerks occur when the muscles (usually in the legs) involuntarily contract very quickly, like a twitch, and it’s thought to serve an important function.
The sudden awakening allows us to check our environment is safe one last time before falling into a (totally vulnerable) sleep. Back in prehistoric times, you might have just nodded off on a rock surrounded by hungry lions, so it’s good to double-check.
Another theory suggests that hypnic jerks are an evolutionary function allowing us (or, our caveman ancestors) to check the stability of our body position before we fall asleep. Like above, the jerk makes sure we’re extra-safe and not in a 50-foot tree or something.
Other ideas claim the reflex is what happens when our brain and body totally relax and switch from active to essentially a state of paralysis.
If you find hypnic jerks particularly annoying, they can be avoided by ensuring your sleep environment is cool, dark, and quiet, and some experts suggest checking you’re not deficient in magnesium, calcium, or iron.
Otherwise, they’re said to decrease with age. And if all else fails, blame your cavemen ancestors for not finding a nice safe cave to snooze in.