Sleep paralysis (SP) is a phenomenon where an individual becomes fully conscious while falling asleep or waking up, but remains temporarily unable to move or speak. This state involves a dissociation between a conscious mind and a completely immobilized body. Episodes are generally brief, lasting from a few seconds to a few minutes, but often involve intense feelings of fear or panic. Tracing the history of this experience reveals a shift from ancient supernatural explanations to a modern understanding of sleep physiology.
Early Cultural Interpretations
Before modern sleep science, the terrifying experience of waking up paralyzed was explained through folklore and mythology across nearly every culture. These early interpretations attributed the phenomenon to malevolent supernatural forces attacking the sleeper. The physical sensations, such as the feeling of being crushed or suffocated, were often given a physical agent.
In ancient Roman and Mesopotamian traditions, the experience was blamed on the Incubus, a demon whose name is derived from the Latin verb meaning “to sit upon.” This entity was believed to physically press down on the sleeper’s chest, causing breathlessness and immobility. The term “night-mare” itself, which existed for centuries before meaning a bad dream, comes from the Old Norse word mara, meaning a “crusher” or female spirit that torments sleepers.
Different cultures developed specific attackers, all sharing the core elements of paralysis and intrusion. In Brazil, the figure is the pisadeira, a creature that steps on the stomach of those who sleep shortly after eating. Newfoundland folklore speaks of the “Old Hag,” a spirit who sits on the victim’s chest, a term used in many English-speaking regions. These narratives established that the experience was viewed as a spiritual or demonic assault, not a medical issue.
The Formal Medical Description
The first recorded instance of sleep paralysis being documented as a clinical entity, separate from superstition, occurred in the 17th century. This transition marked the condition’s “discovery” in a medical context. The earliest detailed case history is attributed to the Dutch physician Isbrand Van Diemerbroeck, whose 1664 work described a patient’s nightly struggle.
Diemerbroeck’s account described a woman who felt pressure and helplessness while awake in bed, a state he referred to as the “Incubus or the Night-Mare.” The documentation was significant because it placed the symptoms within a medical case study framework, though it still used the folkloric term. This established a precedent for viewing the event as an internal affliction rather than an external, demonic attack.
Over the next two centuries, physicians introduced other terms to classify the condition. In 1876, American neurologist Silas Weir Mitchell used “night palsy” to describe the temporary loss of motor function during sleep. The exact phrase currently used was not officially introduced into medical literature until 1928, when Neurologist A.S.K. Wilson first published the term “sleep paralysis.” This progression illustrates the process of separating the physical reality of the experience from its mythical interpretations.
Integrating Sleep Paralysis into Modern Sleep Science
A full understanding of the underlying mechanism of sleep paralysis emerged in the 20th century with the advancement of modern sleep research. Today, sleep paralysis is classified as a parasomnia, a category of undesirable experiences that occur during sleep transitions. This modern classification links the experience directly to the brain’s sleep cycles, particularly the Rapid Eye Movement (REM) stage.
REM sleep is the period when most vivid dreaming occurs, and the brain naturally paralyzes the major voluntary muscles to prevent the sleeper from acting out dreams. This protective paralysis is known as REM atonia. Sleep paralysis occurs when a malfunction in timing causes the brain to wake up and regain consciousness before the REM atonia has completely worn off.
The conscious mind is trapped in a body that remains in a paralyzed state. The muscle inhibition is orchestrated by the brainstem, which uses inhibitory neurotransmitters like glycine and gamma-aminobutyric acid (GABA) to switch off motor neurons. Sleep paralysis is a brief, temporary failure of the brain’s system to synchronize the transition between REM sleep and full wakefulness. This physiological understanding explains why sleep paralysis is associated with other sleep disorders, notably narcolepsy.