Exhausted, you finally crawl into bed, eager for a good night’s sleep. Your body sinks into the mattress, and your mind starts to drift into that soothing space between wakefulness and sleep.

But just as you’re about to fully surrender to slumber, a sudden sensation hits—it’s as if you’re falling through the air, plummeting down with nothing to hold onto. In a split second, your body jerks involuntarily, startling you wide awake.

This sudden and often unsettling movement is known as a hypnic jerk, also called a sleep start. If you’ve experienced it, you’re certainly not alone—up to 70% of people report having this strange jolt as they fall asleep. Despite being so common, it never seems any less abrupt or surprising when it happens.

But why does this happen? What exactly is going on inside the body that causes such an involuntary and sometimes violent jerk right as you’re drifting off to sleep?

While the exact cause remains somewhat of a mystery, there are a few widely accepted theories about what might be triggering these sudden movements.

One prominent theory suggests that hypnic jerks are the result of the body’s nervous system transitioning from wakefulness to sleep. As we relax and start falling into the early stages of sleep, our muscles begin to release the tension built up throughout the day. This relaxation is part of the natural process that prepares us for deep sleep.

However, in some cases, this gradual relaxation can be misinterpreted by the brain as a signal that the body is physically falling or losing control. In response, the brain sends out a burst of neural activity to “catch” the body, resulting in the sudden muscle contraction or jerk that pulls you out of your near-sleep state.

Another theory relates to the downshifting of the nervous system as part of the normal sleep cycle. As the brain transitions from active wakefulness to the slower rhythms of sleep, it sometimes struggles to fully power down smoothly. This can lead to brief misfires in the form of jerky, involuntary movements.

The sensation may be the result of miscommunication between the brain and body, as the nervous system attempts to adjust to the calm, sedative state of sleep.

Interestingly, factors such as stress, anxiety, or fatigue may increase the likelihood of experiencing hypnic jerks. When the body is overtired or stressed, the nervous system may remain on high alert, making it more prone to these sudden, reactive movements.

While hypnic jerks are generally harmless, they can be frustrating, especially if they regularly disrupt your transition into sleep. In most cases, simply recognizing that these sleep starts are a normal part of the sleep process can help ease any anxiety associated with them.