During a typical night, two specific neurotransmitters are released to maintain this state:
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GABA (Gamma-Aminobutyric Acid)
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Glycine
These chemicals effectively switch off the signals from your brain to your skeletal muscles. In a standard cycle, you wake up just as the brain stops producing these chemicals, allowing you to move instantly. However, during an episode of sleep paralysis, your conscious mind wakes up abruptly while the body remains under the chemical influence of REM atonia.
Common Symptoms and Hallucinations
The inability to move is rarely the only symptom. Because the brain is still partially in a dreaming state, episodes are frequently accompanied by vivid, lifelike hallucinations.
| Symptom Category | Description | Common Experience |
| Physical Immobility | Complete inability to move voluntary muscles. | Inability to speak, lift limbs, or turn the head. |
| Incubus Hallucinations | A strong feeling of heavy pressure resting on the chest. | Feeling as though breathing is restricted or someone is sitting on you. |
| Intruder Hallucinations | The intense perception of a hostile presence nearby. | Hearing footsteps, seeing shadows, or sensing someone in the room. |
| Vestibular-Motor Sensations | Illusions of physical movement that are not actually happening. | Feeling like you are floating, flying, falling, or spinning. |
Historical and Cultural Interpretations
Before modern neurology explained REM cycles, cultures around the globe created mythologies to explain this terrifying nocturnal visitor.
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The Old Hag (Newfoundland): Folklore describes an old witch who comes and sits on a sleeper’s chest, leaving them helpless and breathless.
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Kanashibari (Japan): Translating roughly to “bound in metal or leather,” this term describes being tied down by an invisible, spiritual force.
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The Shadow Person (Western Culture): Many modern accounts describe a tall, dark, featureless silhouette standing in the corner of the bedroom or hovering over the bed.
Recognizing that these historical figures are simply cultural interpretations of a shared biological glitch can help remove the fear surrounding the experience.
Primary Triggers and Risk Factors
While occasional episodes can happen to anyone, certain lifestyle choices and health factors increase the likelihood of experiencing this boundary disruption.
