Nightmares or a crippling reality? A review on sleep paralysis

ABSTRACT

Sleep Paralysis (SP) is a state that occurs during the hypnagogic or hypnopompic period in which one finds themselves unable to move despite being conscious. During an episode of SP, the individual experiences muscle atonia, as they would during normal sleep, but they remain conscious. SP is also largely associated with three types of hallucinations – the ‘intruder’, the ‘incubus’, and an out of body experience. It can induce tremendous fear in the individual experiencing it, and cause significant distress. SP has been found to co-occur with chronic conditions such as hypertension and obstructive sleep apnea, and is also associated with Post-Traumatic Stress Disorder. SP has also been depicted in folklore for centuries, leading to various superstitions surrounding its occurrence. Given the sense of fear associated with episodes of SP, as well as the role culture and folklore have played over centuries in using superstition to explain this phenomenon, it is imperative for the primary care physician to empathetically address the patient’s concerns and scientifically, yet sensitively, help dispel myths pertaining to SP. The fact that SP co-occurs with various comorbid conditions, as well as its association with serious psychiatric conditions such as Post-Traumatic Stress Disorder (PTSD) warrants ascertaining a history of its episodes during routine clinical interviews and, if needed, liaising with the appropriate specialists to provide the best therapeutic outcome to the patient.

Introduction

Sleep Paralysis (SP) is a state that occurs during the hypnagogic or hypnopompic period in which one finds themselves unable to move despite being conscious.[1] Recurrent Isolated Sleep Paralysis (RISP) is a kind of Rapid Eye Movement (REM) Parasomnia.[1] It is essentially a transitional dissociative state.[2] During an episode of SP, the individual experiences muscle atonia, as they would during normal sleep, but they remain conscious.[2]

SP is commonly associated with three types of hallucinations, by and large. One type is a hallucination involving the ‘intruder’ – here, the individual may perceive a hostile presence, accompanied by a sense of fear as well as auditory and visual hallucinations (most commonly a shadow or a dark form). The second is the ‘incubus’, which is associated with a sensation of chest tightness that might be painful, a difficulty in breathing, and often a visual hallucination involving a figure that is sitting on top of the chest. And finally, the third type involves vestibular-motor hallucinations, such as the feeling of levitating, spinning, and out of body experiences.[3] The various aforementioned sensations – chest tightness, a difficulty in breathing, a sense of fear – can all cause significant distress to the individual suffering from SP as they can be associated with a fear of dying.[4]

SP carries significant cultural and folklore context. Centuries ago, SP was considered to be the result of nocturnal visits from an evil being that posed a threat to the lives of its victims.[5] In Egyptian folklore, SP was thought to be caused by the ‘Jinn’, a malevolent spirit. In Thai culture, it was believed to be caused by the ‘phi am’, a ghost that haunts victims that are half asleep. Chinese tradition held the ‘ghost of oppression’ responsible for SP. In Newfoundland, stories of the ‘old hag’, a witch that sits on the chest of her victims, percolated as an explanation to episodes of SP.[6]

A study published by Hefnawy et al.[7] in 2024 estimates the global prevalence of SP to be 30%. Its prevalence in many countries is yet to be officially documented. Given its integration with various cultures as a supernatural phenomenon, as discussed earlier, it is imperative for physicians to understand SP from a medical perspective, as well as from a sociocultural perspective, and utilize existing information to not only identify it in their patients during routine office visits, but also effectively manage it and allay their patients’ anxiety. The family physician is the point of primary care for patients, and can help dispel myths and enhance the patients’ understanding of the science behind SP.

Methods

A meticulous and comprehensive search was carried out using ‘Google Scholar’ and ‘PubMed’ as databases. The keywords included in this search were ‘Sleep Paralysis’, ‘parasomnias’, ‘isolated sleep paralysis’, ‘nighttime paralysis’, ‘sleep disturbances’, ‘REM’. Articles were chosen after a thorough examination of titles as well as abstracts. Case reports were eliminated from the review. The time period of articles that were analyzed was from 2014 to 2024.

Results

Studies published between 2014 and 2024 were included in this article. The findings from this review have been detailed under various appropriate categories and sub-headings in the ‘Discussion’ section of this article.

Discussion

Demographics of SP

A majority of isolated SP or Isolated Sleep Paralysis (ISP) (SP in the absence of other sleep disorders) cases are first reported in in early childhood, with their frequency peaking in adolescence.[8] While more initial surveys found there to be no gender disparity in the frequency of SP, more recent studies are demonstrating a higher episode recurrence amongst females.[8]

A study conducted by Rauf et al.,[9] published in 2023, surveyed 6,811 study participants. They found that females were more likely to experience ISP than males, as were younger participants. Non-white participants were more likely to experience ISP than White participants. Additionally, the study subjects who reported having experienced ISP were also found to have a longer sleep onset latency (the amount of time that occurs between going to bed and sleep occurring).

Associated conditions

SP has been demonstrated to be associated with psychological stress,[10] an erratic sleep schedule and sleep deprivation and other sleep disturbances.[9]

The following comorbid medical conditions, too, have been associated with SP:

Hypertension: Likely due to a potential adrenergic dysfunction brought about by SP.[11]

Alcohol use: The consumption of and withdrawal from alcohol can affect the REM sleep cycle, which can result in SP.[12]

Obstructive Sleep Apnea (OSA): This has been found to be linked to episodes of SP as well.[11]

Narcolepsy: One symptom of narcolepsy is an abnormal REM sleep manifesting in the form of SP.[11]

Post-Traumatic Stress Disorder (PTSD), childhood sexual abuse, panic disorders and anxiety disorders: Research has implicated SP as a consequence of all of these variables.[5,13]

Pathophysiology of SP

A dream occurs during the REM phase of the sleep cycle, during which there is muscle atonia. As there is a tendency to have our most emotional dreams during REM sleep, the brain keeps us temporarily paralyzed in order to prevent the physical enactment of these dreams. This is brought about by skeletal muscle tone suppression by the pons and ventromedial medulla via the neurotransmitters, gamma-aminobutyric acid (GABA) and glycine, which play an inhibitory role to the motor neurons within the spinal cord. SP occurs when one awakens and becomes aware while remaining under REM paralysis.[5]

It is imperative to note that the parietal lobe of the brain is responsible for sensation and perception – it integrates sensory inputs to the visual system, likely implicating it in the perception of intruder hallucinations.[14]

The brainstem activation of the amygdala is likely implicated in the sensing of an intruder in the room, the fear accompanying the intruder sensation, and the increased awareness of potential danger. The subthalamo-amygdala pathway then ensures that in this perceived moment of danger, the body mounts a reaction without an in-depth analysis by the sensory cortex.[5]

While variations in the circadian rhythm gene PER2 have demonstrated an increase in the odds of SP,[10] genetic studies are relatively lacking, and wider genome studies are yet to be performed.[15]

Clinical features of SP

As mentioned earlier, SP is commonly associated with three types of hallucinations, by and large. One type is a hallucination involving the ‘intruder’ – here, the individual may perceive a hostile presence, accompanied by a sense of fear as well as auditory and visual hallucinations (most commonly a shadow or a dark form). The second is the ‘incubus’, which is associated with a sensation of chest tightness that might be painful, a difficulty in breathing, and often a visual hallucination involving a figure that is sitting on top of the chest. And finally, the third type involves vestibular-motor hallucinations, such as the feeling of levitating, spinning, and out of body experiences.[3] The various aforementioned sensations – chest tightness, a difficulty in breathing, a sense of fear – can all cause significant distress to the individual suffering from SP as they can be associated with a fear of dying.[4]

The 3^rd edition of the International Classification of Sleep Disorders (ICSD-3) outlines the following criteria for RISP, all of which must be met:[16]

1. An inability to move the trunk and all of the limbs at the onset of sleep or upon awakening; this inability is recurring.

2. Each of these episodes lasts from a few seconds to a few minutes.

3. These episodes cause significant distress to the individual, including bedtime anxiety or a fear of sleeping.

4. This disturbance is not better explained by another sleep disorder, mental disorder, medical condition, medication, or substance use.

Evaluation of the patient

After assessing the history of the patient as well as matching the symptoms to the ICSD-3 criteria mentioned previously, the physician should conduct a physical exam. Sleep studies might reveal the following findings during an episode of SP: REM, muscle atonia, a decrease in respiratory muscle activity, and an increase in heart rate.[17]

Treatment of SP

SP, when it occurs in isolation, has not been shown to impair daytime functioning, and is generally self-limited. Since sleep deprivation and an erratic sleep schedule have been implicated in the development of SP, it is important to educate the patient regarding sleep hygiene, ensuring they get adequate sleep at night, and maintain a regular sleep-wake schedule as far as possible.[15]

If patients have bed partners, then their bed partners can be instructed to touch the patient if they hear low vocalizations before awakening.[15]

Pharmacological therapy is not indicated for SP. However, reports have established that in patients of narcolepsy, treatment with tricyclic as well as other antidepressants has been shown to not only improve cataplexy, but also reduce the number of episodes of SP.[15]

Cognitive Behavioral Therapy (CBT), too, may be considered to cope with the fear and distress resulting from the hallucinations.[15]

Conclusions

The family or primary care physician plays an integral role in the lives of their patients – from addressing a wide variety of health conditions by providing comprehensive healthcare, to educating not only the patient, but also the larger community regarding health maintenance, as well as providing care with utmost cultural sensitivity.[18]

Given the sense of fear associated with episodes of SP, as well as the role culture and folklore have played over centuries in using superstition to explain this phenomenon, it is imperative for the primary care physician to empathetically address the patient’s concerns and scientifically, yet sensitively, help dispel myths pertaining to SP.

The information compiled in this article provides an overall understanding of SP, and the most effective way to tackle it. The fact that SP co-occurs with various comorbid conditions, as well as its association with serious psychiatric conditions such as PTSD warrants ascertaining a history of its episodes during routine clinical interviews and, if needed, liaising with the appropriate specialists to provide the best therapeutic outcome to the patient.

Author contributions

The author has accepted responsibility for the entire content of this manuscript and approved its submission.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.