Hypnic jerks, also known as sleep starts, are sudden muscle contractions that occur when falling asleep. They often feel like falling and can jolt you awake. According to the Sleep Foundation, they typically affect one side of the body, like the left arm or right leg, though their exact cause remains unclear.

One theory suggests hypnic jerks stem from the brain’s startle response, mistaking muscle relaxation for falling. Another idea links them to dream-like imagery, with the brain reacting physically to dreams. “Hypnic jerks…begin in the same area of your brain that controls your startle response.”

Certain factors may increase their likelihood, such as caffeine or nicotine consumption, stress, lack of sleep, or late-night exercise. “Stimulants such as caffeine and nicotine can play a particular role since they remain in one’s system for hours after consumption.”

While unsettling, hypnic jerks are common and not harmful, affecting up to 70% of people. Practicing good sleep hygiene, reducing caffeine, and managing stress can help minimize their occurrence.

Though the sensation is uncomfortable, it’s a natural part of the sleep process for many people.