Exhausted, you finally lie in bed, sinking into what you hope will be a deep sleep, when suddenly your body feels like it’s falling—and then jerks abruptly to stop that fall. Of course, you immediately wake up, startled and wondering what just happened.
No matter how often it occurs, you never quite get used to it. The sensation is always sudden, jarring, and often a bit terrifying. That strange feeling of falling, followed by an involuntary jolt, can leave you wide awake again.
If this has happened to you, you’re not alone. In fact, up to 70% of people experience this phenomenon, known as hypnic jerks (or sleep starts). While scientists don’t yet fully understand why these sudden jolts occur, there are a few leading theories.
The Science Behind Hypnic Jerks
Although there’s no single, definitive explanation, two main theories shed some light on why hypnic jerks happen.
1. The nervous system slowing down
One theory suggests that hypnic jerks are linked to the natural slowing of the nervous system as the body transitions into sleep. As your breathing and heart rate slow and your body temperature drops, muscle tone also decreases. These physiological changes may trigger sudden muscle contractions as the body adjusts to this state of relaxation.
2. The brain’s “falling” misinterpretation
Another theory proposes that hypnic jerks occur when the muscles relax too quickly, causing the brain to misinterpret the sensation as falling. In response, the brain sends signals to the muscles to contract—essentially a reflex to “protect” you from an imagined fall.
Researchers also believe that certain external factors can make hypnic jerks more frequent or intense. These include caffeine, nicotine, stress, sleep deprivation, and the use of stimulant medications such as Adderall or Ritalin.
