Many people feel a sudden jolt just as they are falling asleep. Doctors call this a hypnic jerk, also named a sleep start or hypnic myoclonus. It is a brief, involuntary muscle contraction that can come with a feeling of falling or a quick startle. It happens during the first light moments of sleep, when the body moves from wake to sleep.
Specialists think this reflex starts in brain areas that also control the startle response. One leading explanation is simple, when your muscles relax at sleep onset, the brain sometimes misreads that normal relaxation as a fall and sends a quick signal to contract muscles. This has not been proven in full detail, but it fits what people report.
These twitches are very common in healthy people and are not a disease. Good evidence shows that most adults experience them at least once in life. They are more likely when stress is high, after caffeine or nicotine, or when sleep is short.
Sleep follows a usual order, light non-REM sleep first, then deeper non-REM sleep, then REM sleep with vivid dreams. Hypnic jerks occur at the beginning of sleep, mainly in the first light stage, not in deep sleep.
Some scientists have proposed an evolutionary idea, that an early primate sleeping in trees might benefit from a reflex that checks body position when muscles relax. This is a hypothesis only. It is consistent with the view that some early human ancestors often slept in trees, yet there is no direct proof that hypnic jerks evolved for this purpose.
If the twitches are rare, no treatment is needed. If they are frequent and disturb sleep, lowering stimulants, reducing stress, and keeping a regular sleep schedule can help. A doctor can check for other movement disorders if needed.
The idea that the brain mistakes normal muscle relaxation for a fall is a leading explanatory model, not proven.
The evolutionary link to tree sleeping is a plausible hypothesis, not established fact. Arboreal sleeping in some early hominins is discussed by experts, but direct evidence that hypnic jerks evolved for fall prevention is lacking.
Hypnic jerks are an underestimated sleep motor phenomenon in parkinsonism – 2016
Clinical paper describing hypnic jerks as brief muscle contractions at sleep onset. It notes they occur across ages and sexes and estimates a prevalence around 60 to 70 percent in the general population.
Hypnic Jerks, Major Depressive Disorder, and Vitamin D Deficiency – 2023
Review and case material that defines hypnic jerks and states they are experienced by up to about 70 percent of adults at some point, while emphasizing their benign nature and possible triggers.
Hypnic Jerk, Why You Twitch When You Sleep – 2025
Consumer health overview explaining that hypnic jerks occur as you transition into sleep, often with a falling sensation. It presents a leading explanation, complete muscle relaxation being misread by the brain as a fall, and lists common triggers like stress, caffeine, and sleep loss.
Were Our Ancestors Sleeping in Trees 3 Million Years Ago? – 2018
Reports expert views that Australopithecus probably often slept in trees for safety. Useful background for the idea that some hominin ancestors were arboreal sleepers.
Why You Sometimes Feel Like You’re Falling And Jerk Awake When Trying To Fall Asleep – 2014
Quotes psychologist Frederick Coolidge of the University of Colorado proposing the evolutionary hypothesis, a reflex to prevent a fall from trees, and explicitly notes that direct evidence is lacking. This frames the idea as speculative.
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