Clinical reviewLifetime prevalence rates of sleep paralysis: A systematic review
Introduction
Sleep paralysis (SP) is characterized by a discrete period of time during which voluntary muscle movement is inhibited, yet ocular and respiratory movements are intact and ones sensorium remains clear.1 These episodes can occur when falling asleep or upon awakening, and are most likely to happen when individuals sleep in a supine position.2 Some of the more notable aspects of SP are the vivid hypnogogic (sleep onset) or hypnopompic (sleep offset) hallucinations that often accompany episodes. These potentially frightening experiences have been interpreted in a number of culturally- specific contexts, with variegated spiritual and supernatural explanations ranging from witchcraft and malevolent spirits to extra-terrestrials.3 Contemporary medical explanations for the genesis of SP are not so colorful, with sleep studies locating SP's genesis in a perseveration of REM activity into normal sleep transitions.1
Episodes of SP have been linked with conditions such as narcolepsy, hypertension, and seizure disorders, but are also associated with a general lack of sleep, sleep disturbances, jet lag, student status, African descent, and shift work.4, 5, ∗6. When SP occurs in otherwise healthy individuals it is termed isolated SP. Neither SP nor isolated SP episodes are currently recognized as codable diagnoses. However, the International Classification of Sleep Disorders 2nd Edition1 includes recurrent isolated SP as a diagnostic possibility, and these same symptoms could be classified as a parasomnia not otherwise specified in the Diagnostic and Statistical Manual of Mental Disorders, 4th edition (DSM-IV).7
Section snippets
Fear and SP
SP episodes are often experienced as frightening. Cheyne et al.8 found that 90% of a student sample and 98% of a web-based sample reported fear, and clinically significant levels of fear were found in 69% of Sharpless et al.'s9 psychiatric sample. These high rates of fearfulness are in contrast to the relatively lower rates experienced during normal dreaming, where it occurs approximately 30% of the time.10
The fear associated with SP appears to arise not only from individual reactions to
SP and psychopathology
Along with the above-mentioned relationship to narcolepsy and other medical conditions, several lines of evidence imply that SP may be related to certain psychiatric disorders. SP has been associated with dissociative phenomena,13 but it has probably been most frequently assessed within the context of the anxiety disorders in general6 and with panic disorder14 and post-traumatic stress disorder∗9., ∗15. in particular. Elevated rates of anxiety sensitivity have also been found in individuals
Present study
The objective of the present study is to comprehensively survey the available literature in order to calculate lifetime prevalence rates for certain subgroups. We predict that rates of SP will be lower in general population samples than in student samples, and that the highest rates will be found in psychiatric patients. We also predict that lifetime rates of SP will be higher in individuals of African descent. Exploratory analysis of SP rates according to gender will also take place. However,
Method
A key word literature search of “sleep paralysis,” “isolated sleep paralysis,” and “parasomnia not otherwise specified” was conducted using MEDLINE (1950–present) and PsycINFO (1872–present) databases on May 1st, 2010. MEDLINE yielded 314 abstracts and PsychINFO yielded 370. All English and Spanish language abstracts were initially examined by the first author. Additional searches through the reference lists of identified articles also took place, and two additional articles were suggested by a
Results
The 35 articles included in the analyses can be found in Table 1. They span 5 decades of research and represent a truly international and cross-cultural sample. Regarding assessment modality, self-report measures were clearly favored, and were used in 68.6% of the studies.
Discussion
In conclusion, we have reviewed the available literature on lifetime episodes of SP and have found it to be a fairly common experience. Although occurring in less than 8.0% of the general population, it is much more frequent in students and psychiatric patients, and the difference between these latter two groups is surprisingly small. Reasons for these higher prevalence rates are unclear, but it is possible that both groups experience regular sleep disturbances, a factor making SP episodes more
Practice points
Knowledge of sleep paralysis prevalence rates imply that:
- 1)
sleep paralysis should be assessed more frequently in routine clinical care situations, especially when working with students, psychiatric patients, and patients with panic disorder.
- 2)
sleep paralysis should be assessed more frequently in minority patients
- 3)
experiences of sleep paralysis are relatively common, and they should be appropriately normalized when disclosed by patients.
Research agenda
In the future, we need to not only identify patients with a lifetime history of sleep paralysis, but also:
- 1)
more uniformly report relevant demographic variables (e.g., ethnicity, age, psychiatric status) of patients with sleep paralysis
- 2)
better determine the typical frequency of sleep paralysis episodes
- 3)
better determine the typical levels of fear, distress, and clinical impairment experienced both during sleep paralysis episodes and as a result of such episodes
- 4)
determine the most appropriate
Acknowledgements
We would like to thank the authors who responded to our emails with additional clarifications on their published studies as well as the anonymous reviewers. This work was supported in part by a grant (NIMH R01 MH 070664) held by Jacques P. Barber.
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