ワイ「そろそろ寝るか…（ｵﾒﾒﾄｼﾞｰ）」体「ビクンッ ビクンッッ」ワイ「きたきた！！」
A trending online thread captures the humorous anticipation of a common pre-sleep phenomenon: the 'hypnic jerk.' Users describe their bodies giving a sudden twitch just as they're about to fall asleep, exclaiming, 'There it is!' with amusement.
Medically known as 'hypnagogic myoclonus,' this involuntary twitch is a normal physiological occurrence experienced by healthy individuals worldwide. The thread has become a hub for relatable 'been there, done that' moments, with comments like 'I totally get it!', 'It feels like falling into space,' and 'I'm so tired today, it's bound to happen,' creating a shared experience of this universal bodily quirk.
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Hypnic Jerk (Hypnagogic Myoclonus)
Hypnic jerk, also known as hypnagogic myoclonus or a sleep start, refers to the involuntary twitching or jerking of a part or the entire body that occurs just as a person is falling asleep, especially when consciousness is still lingering. Commonly described as 'jerking' or 'sleep twitch,' it is a very common physiological phenomenon that most people experience at least once. It is estimated that approximately 60-70% of adults experience it, making it far from rare. This phenomenon is a type of myoclonus, which involves sudden muscle contractions. Unlike pathological myoclonus associated with neurological disorders like epilepsy, hypnic jerks in healthy individuals are generally not a cause for concern. Symptoms often include a large, sudden movement of the legs, arms, or sometimes the whole body for a few seconds, occasionally accompanied by a mild sensation of falling or floating. One hypothesis suggests that the brain misinterprets information, for example, mistaking the body's relaxation for 'falling to the ground,' as a cause of this phenomenon. It is reported to occur more frequently when stress or fatigue accumulates throughout the day, after consuming stimulants like caffeine, or during periods of irregular sleep patterns. It is particularly common among shift workers, those experiencing jet lag, or individuals who go to sleep immediately after exercise. While the phenomenon itself subsides within a few seconds and rarely has adverse health effects, if it occurs too frequently or is accompanied by pain, it might rarely indicate another sleep disorder or neurological issue, warranting consultation with a specialist.
Sleep Stages
Human sleep cycles through two main stages, 'REM sleep' and 'NREM sleep,' over the course of a night. NREM sleep is further subdivided into three stages (Stage 1 to Stage 3, formerly including Stage 4) based on depth of sleep. Hypnic jerks, the topic of this content, are known to occur precisely during the transition between sleep stages, specifically when transitioning from wakefulness to NREM Stage 1. Stage 1 is the lightest stage of sleep, characterized by drowsiness, where brainwave patterns still show characteristics of both wakefulness and sleep, indicating an unstable state. During this stage, the brain is not completely inactive, and muscles still maintain a certain degree of tension. Hypnic jerks are sometimes interpreted as a 'malfunction' between the brain and body systems, occurring when the smooth transition from wakefulness to sleep is disrupted. Specifically, it is believed that while neural circuits maintaining wakefulness are still active, neural circuits promoting sleep begin to work, leading to a temporary imbalance in information transmission between the two, which stimulates motor nerves and causes involuntary muscle contractions. In an adult, these sleep stages typically repeat in cycles of approximately 90 minutes throughout the night, but hypnic jerks usually occur at the beginning of the first sleep cycle, within minutes of falling asleep. Understanding sleep stages is fundamental not only for phenomena like hypnic jerks but also for the diagnosis and treatment of various sleep-related disorders such as insomnia, sleep apnea syndrome, and REM sleep behavior disorder.
Transition between Wakefulness and Sleep
For humans, the transition from wakefulness to sleep, or from sleep to wakefulness, is not a simple process of merely 'opening or closing the eyes.' It is a very delicate state transition where multiple neurotransmitters and neural circuits in the brain intricately cooperate to dynamically change consciousness and bodily functions. Hypnic jerks are particularly notable as phenomena occurring precisely at this 'boundary between wakefulness and sleep.' During this transitional period, the brain is not entirely switched into sleep mode; rather, remnants of wakefulness and preparations for sleep coexist. For example, the activity of neurotransmitters that promote brain arousal (e.g., orexin, serotonin, noradrenaline) gradually decreases, while the activity of neurotransmitters that promote sleep (e.g., GABA) becomes dominant. An temporary imbalance between these two is thought to lead to involuntary stimulation of motor nerves. Specifically, one prominent hypothesis suggests that certain parts of the brain's motor cortex and brainstem, still under the influence of the active arousal system, send 'misplaced signals' at the moment they should be issuing commands for muscle relaxation, resulting in sudden muscle contraction. Furthermore, phenomena such as hypnagogic hallucinations and sleep paralysis can also be triggered by a confusion of brain functions at this boundary between wakefulness and sleep. These phenomena are characterized by occurring in a 'gray zone' where the brain has not fully disengaged from the waking state or has not fully entered the sleeping state. Understanding this boundary provides an essential perspective for creating an environment conducive to comfortable sleep, effective use of sleep aids, and fundamental elucidation and development of treatments for sleep disorders. In essence, the boundary between wakefulness and sleep is not just a temporal marker, but a critical phase condensed with the physiological and neurological dynamics of the brain.