Surveys of normal subjects indicate that isolated sleep paralysis occurs at least once in a lifetime in a substantial proportion of otherwise normal subjects. Incidence of sleep paralysis in young adult Nigerians was 14 (Oluwole 2010). The prevalence of sporadic sleep paralysis reached 35.5% in Nigerian students (Ohaeri 1997), 40% in Japanese college students, and 37% in Chinese undergraduates (Wing et al 1994). Peak onset was between 17 and 19 years of age, with no sex difference (Wing et al 1994). Twenty percent of the cases were familial. Eight percent of Chinese older than 70 years of age reported isolated sleep paralysis (Wing et al 1999). Cheyne and colleagues found that 30% of 870 university students reported at least 1 incident; 10% reported 3 or more isolated sleep paralysis episodes in their lifetime (Cheyne et al 1999). In a sleep survey of 8162 citizens, the cumulative experience of sleep paralysis was 39.6%, with the onset peaking at 16 years of age (Kotorii et al 2001). However, Ohayon and colleagues found strikingly lower rates: 6.2% of 494 otherwise normal subjects reported at least 1 episode of isolated sleep paralysis in their lifetime, with 4% reporting less than 1 episode per month and 0.8% more than 1 per week (Ohayon et al 1999). Such discrepancies in prevalence rates of isolated sleep paralysis are at least in part explained by different population characteristics and expressions used in questionnaires. A recent systematic literature review aggregating 36,533 subjects across different studies found that 7.6% of the general population, 28.3% of students, and 31.9% of psychiatric patients reported at least one sleep paralysis event (Sharpless and Barber 2011). Cultural and genetic factors may also be relevant. Isolated sleep paralysis occurs more frequently among African Americans than Caucasians, especially when associated with panic disorders and agoraphobia (Friedman and Paradis 2002). Indeed, in the Afro-American population, panic disorder, anxiety sensitivity, and life stress were found associated with sleep paralysis (Ramsawh et al 2008). The prevalence of sleep paralysis did not differ between Japanese and Canadians (Fukuda et al 2000), but Kuwaiti and Sudanese had a greater prevalence than American college students (Awadalla et al 2004).
Approximately 41% of narcoleptic patients frequently suffer from sleep paralysis (Han et al 2001), which is sometimes associated with rhythmic movement disorder (Pizza et al 2010). Sleep paralysis and cataplexy occur more often in patients affected with Wilson disease, suggesting an altered REM sleep function (Portala et al 2002). Myotonic dystrophy type 1 patients with subjective sleepiness also reported more cataplexy-like and sleep paralysis symptoms (Laberge et al 2009). Sleep paralysis is not, however, a feature of parkinsonism with excessive daytime sleepiness (Baumann et al 2005).