As you fall asleep, your body shifts from wakefulness into a relaxed state.

Your muscles loosen, breathing slows, and your brain activity begins to change. In some cases, the brain misinterprets this sudden relaxation as a loss of control, similar to falling. It responds by sending a quick burst of signals to your muscles, causing a sudden jerk that can wake you up.

This reflex is known as a hypnic jerk, and it is often accompanied by a vivid sensation of falling, or a brief dream-like image just before sleep.

Hypnic jerks have been observed for over a century, but scientists began to understand them more clearly in the mid-20th century as sleep research advanced.

Even today, their exact cause is not fully settled.