This movement, known as a hypnic jerk or sleep start, is common—up to 70% of people experience it. Despite being frequent, it’s always startling when it happens. But why does this occur?
One theory is that it’s the result of the nervous system transitioning from wakefulness to sleep. As the muscles relax, the brain might misinterpret this and trigger a jerking response to “catch” the body.
Another theory involves the nervous system downshifting during sleep. Misfires in brain activity can lead to involuntary movements, making the body jerk as it adjusts to sleep.
Stress, anxiety, or fatigue can increase these movements. While frustrating, hypnic jerks are usually harmless and part of the natural sleep process.
