Drained and exhausted, you are finally in bed sinking into what will be a deep sleep, when your body starts ‘falling’ and it suddenly jerks to stop that ‘fall’. You will of course wake up immediately and wonder where it came from.
And no matter how many times happens, you never get used to it, it never feels less abrupt, and it never gets any better. The falling sensation can even be terrifying, which ends with an involuntary, sudden, and shocking jump.

If this has happened to you at least once, you are not alone. In fact, up to 70% of people experience this phenomenon, also known as hypnic jerks (or sleep starts). Nobody really knows what causes these sudden jerks for sure.
Although a solid explanation for how and why hypnic jerks happen hasn’t been offered, there are two theories that suggest some reasons for this. One states that the hypnic jerks happen as a result of the natural downshifting of the nervous system as you’re falling asleep.