A hypnic jerk, also called a sleep onset, is a sudden, involuntary muscle movement that occurs during sleep. It often feels like a jolt or a falling sensation and can temporarily wake you up. The phenomenon is common and usually harmless, but it may make you wonder why it happens.

What is a hypnic jerk?

A hypnic jerk occurs during the first stage of sleep, when your body transitions from wakefulness to rest. As your muscles relax, your body may jerk unexpectedly, and this jerk may be accompanied by a falling sensation, a loud popping sound, or a flash of light in your mind.

Possible causes of a hypnic jerk

Although the exact cause isn’t fully understood, there are several theories and contributing factors:

Muscle relaxation reflex
When your muscles relax too quickly as you transition to sleep, your brain may mistake this for a falling sensation. To protect you, it sends a signal to “catch yourself,” causing a sudden muscle spasm.