Hypnic jerks are a natural part of the sleep process, so many doctors believe it's just the body "twitching" as it slowly shuts down for rest.
Another leading theory, Breus says, is that it happens when the body goes through the first sleep stage too quickly, because it's so exhausted. During the first stage of sleep, which usually only lasts a few minutes, your breathing and heart rate slows down and your sleep is still very light. If the body speeds through this stage and "shuts down" too fast, it might trigger the brain to think your vitals are actually failing and in response, it jerks the body awake. "It might be a kind of protective mechanism, but we really aren't sure because it's difficult to study," Breus says.