Sleep is a basic requirement of life and an important indicator of health. Not
surprisingly, the relation between sleep and psychopathology appears to be quite
complex. Certain problematic symptoms are known to impede sleep (e.g., chronic
worry and depressive ruminations), whereas others occur in the context of sleep (e.g.,
sleep terrors, nocturnal panic attacks, and sleep paralysis). The focus of this article is
on sleep paralysis (SP), a potentially frightening experience that can occur during the
onset of sleep or upon awakening.
Sleep paralysis is characterized by a period of time during which voluntary muscle
movement is inhibited, yet ocular and respiratory movements are intact and the
senses remain relatively clear (American Academy of Sleep Medicine [AASM],
2005). When SP occurs in otherwise healthy individuals, it is termed isolated sleep
paralysis(ISP). In the Diagnostic and Statistical Manual of Mental Disorders, Fourth
Edition Revised (DSM-IV-TR; American Psychiatric Association, 2000), ISP is
classified as a parasomnia not otherwise specified. Through the use of sleep studies,
inroads have been made towards understanding some of the origins of SP and ISP.
These conditions have been associated with rapid eye movement (REM) activity and
are often considered to result from a perseveration of aspects of REM sleep into
sleep transitions (AASM, 2005).
Prevalence Rates
A review of the SP and ISP literature indicates these episodes are not uncommon,
but their prevalence rates are quite variable across studies. For instance, the lifetime
prevalence rate in nonclinical samples ranges from 2.2% (Ohayon & Shapiro, 2000)
to 39.6% (Kotorii et al., 2001), and in clinical samples ranges from 7.4% (Ohayon &
Shapiro, 2000) to 50.0% (Bell, Hildreth, Jenkins, & Carter, 198
. Given the wide
range of SP assessment methods (i.e., various self-report instruments and structured
interviews), differing definitions of sleep paralysis, and different levels of reported
detail, it is perhaps not surprising that the interstudy variability is so pronounced.
Further, differentiation between SP and ISP is obscured by the fact that individuals
with other conditions (especially narcolepsy, seizure disorder) were often not
documented and/or excluded from the samples, a factor making diagnosis of ISP
impossible. Few systematic demographic correlates have been found, yet some
evidence exists that persons of African descent may experience higher rates of SP
than those of European descent (e.g., Paradis, Friedman, & Hatch, 1997).
Associated Features
Sleep paralysis and isolated sleep paralysis episodes are often accompanied by
hallucinations that can be extremely vivid and sometimes disturbing. Cheyne,
Rueffer, and Newby-Clark (1999) found that these hallucinations reliably cluster
into three main categories: intruder, incubus (nocturnally assaulting demon), and
vestibular–motor. These categories are consistent with known aspects of low-level
neural mechanisms found in REM neurophysiology. For instance, the first two
categories are consistent with threat-activated vigilance systems that typically
involve the amygdala, and the third appears to be associated with REM stages that
Fearful Isolated Sleep Paralysis 1293
Journal of Clinical Psychology DOI: 10.1002/jclp