Hypnic jerks: Understanding sleep-onset muscle spasms

The phenomenon of jerking while falling asleep, known as a hypnic jerk or nocturnal myoclonus, is a common and generally harmless occurrence. According to the National Sleep Foundation, approximately 70% of people have experienced these involuntary muscle spasms, which typically affect a single limb.

While the exact cause is not definitively known, scientists suggest several theories. One possibility involves a malfunction between the nerves of the reticular brainstem, which controls startle responses. Another theory posits that as muscles relax during sleep onset, the brain misinterprets the sensation as falling, triggering a corrective muscular contraction.

Certain factors may increase the frequency of these jerks, including the use of stimulants such as caffeine and nicotine, which can disrupt sleep patterns.