Few things ruin a night’s sleep like the sudden, panicked sensation of jerking awake — a feeling many have experienced. Often, it’s accompanied by the sensation of falling, similar to a dream where you’re plummeting but wake up before hitting the ground.

This phenomenon is known as hypnic jerks or sleep starts — sudden muscle contractions that typically affect one side of the body. Though researchers aren’t entirely sure why they happen, theories suggest they stem from the brain’s startle response. As your muscles relax during sleep, your brain might mistakenly think you’re actually falling, prompting a physical jolt. Another idea is that the brain blurs dream imagery with reality, causing a reaction.