Sudden body jerks that occur while falling asleep are called hypnic jerks (also known as sleep starts). According to scientists, about 70% of people experience hypnic jerks at least once in their lifetime.
As a person falls asleep, the body enters a transitional state between wakefulness and sleep, known as the hypnagogic phase of consciousness. Just before the jerk occurs, the brain may send a sudden signal to wake up, as if something unexpected is happening — for example, muscle relaxation or a false sensation of falling.
These involuntary movements can feel like a shock, twitch, or sudden jump of the body.
The exact cause of hypnic jerks is not fully understood, but researchers suggest several possible explanations.
In some cases, night-time jerks may be linked to medical conditions, such as:
Restless leg syndrome (RLS) – a neurological condition that causes an uncontrollable urge to move the legs, often accompanied by sudden muscle movements.
Brain injuries or neurological disorders, which can disrupt normal brain signaling and trigger involuntary muscle contractions.
Psychological factors also play an important role. People who experience high levels of stress, anxiety, or mental overload are more likely to have hypnic jerks. In such cases, these movements may act as a protective mechanism of the brain, helping it transition from an active mental state into sleep.
For individuals who constantly overthink or remain mentally alert, the brain may use these sudden jerks as a signal that it is finally time to relax and fall asleep.
In simple terms, hypnic jerks are a normal physiological reaction during the process of falling asleep and, in most cases, are harmless.
