Sleep paralysis in adolescents: The ‘a dead body climbed on top of me’ phenomenon in Mexico
*Alejandro Jiménez-Genchi, MD, Servicios Clínicos, Instituto Nacional de Psiquiatría Ramón de la Fuente Muñiz, Calz, México-Xochimilco 101, Col. San Lorenzo Huipulco C. P. 14370, Tlalpan, México DF, México. Email: firstname.lastname@example.org
Aims: The aim of the present study was to evaluate the prevalence and characteristics of sleep paralysis in adolescents using a folk expression.
Methods: Three hundred and twenty-two adolescents (mean age, 15.9 ± 0.88 years; 66.8% female) from three high schools in Mexico City completed both a self-reported questionnaire, including a colloquial definition of sleep paralysis and the Epworth Sleepiness Scale.
Results: A high proportion of the adolescents (92.5%) had heard about the ‘a dead body climbed on top of me’ expression and 27.6% of them had experienced the phenomenon. Sleep paralysis was present in 25.5% while the prevalence rate for hypnagogic/hypnopompic hallucinations was 22%; 61% had experienced ≥2 episodes in their lifetime. The mean age of onset was 12.5 ± 3 years. Sleepiness scores for the subjects who had experienced at least one event were not significantly different from subjects who had not experienced any. In 72% of cases, the episodes were composed of both sleep paralysis and hallucinations while 20.2% consisted of only sleep paralysis and 7.8% of only hallucinations. The number and characteristics of events were not significantly different between adolescents with only one episode and those with two or more episodes.
Conclusions: The characteristics of the ‘a dead body climbed on top of me’ phenomenon suggest that is identical to sleep paralysis and a frequent experience among Mexican adolescents. During adolescence, sleep paralysis seems to be a recurrent phenomenon frequently accompanied by hallucinatory experiences.
SLEEP PARALYSIS (SP) consists of a transient period of inability to perform voluntary movements at the onset of sleep or upon awakenings during the night or in the morning.
Although the high frequency of SP in narcoleptic patients is well recognized,1 research on the prevalence of isolated SP in the general population has yielded conflicting results. Early epidemiologic studies found lifetime prevalence rates of 4.7–16.3% among US medical students.2–4 In contrast, subsequent studies reported higher rates in different groups of young adults: 40% in Japanese,5 37% in Chinese,6 41% in Afro-American,7 and 26% in Nigerian.8 In contrast, studies that have investigated the prevalence of SP in representative samples of the general population have found a rate of 11.3% among residents of Monterrey City, Mexico,9 and 6.5% in a sample of Italian and German subjects.10
One explanation for the discrepant rates is related to the wording of questions used to investigate SP.11 Some cultures have colloquial expressions to name the SP experience (e.g. kanashibari, ‘ghost oppression’, ‘the old hag’), which have been argued to influence the recognition of SP.5,6,12 Studies that have used folk expressions have obtained higher figures5,6,12 than investigations using clinical descriptions.2–4,9,10 A memory bias might also play a role considering that approximately half of the subjects experience 1–3 episodes of SP during their lifetime, with the first one appearing between 14 and 17 years.5,6 In fact, even though SP often appears in adolescence, few studies on it have included adolescents10 and thus the prevalence rate in this age group is unknown.
While epidemiologic data about SP in Mexico have been obtained from adult population through brief clinical definitions of SP,9 Mexican people use the expression ‘a dead body climbed on top of me’ (se me subió el muerto) to describe a phenomenon that seems to be the equivalent of SP.13
On the basis of this knowledge, the aim of the present study was to evaluate the prevalence and characteristics of SP in adolescents using a folk expression.
Subjects and procedures
Adolescents were students from three high schools in Mexico City. In the first phase, one researcher, or research assistant, obtained permission from the school authorities to attend the classrooms and give the students (n = 398) a brief explanation about the research and to invite them to participate in it. Few students refused to collaborate (n = 13). Those who chose to participate (n = 385) were asked to take home the informed consent form, sign it themselves and have it signed by their parents or caregivers. In the second phase, participants were required to return the signed consent form and to complete both an SP questionnaire and the Epworth Sleepiness Scale (ESS; n = 358; response rate = 90%). Finally, 36 subjects were excluded because they were ≥18 years old.
The research project was approved by the Boards of Ethics and Research from the National Institute of Psychiatry Ramón de la Fuente.
The design of the SP questionnaire was based on the questionnaires used by Fukuda et al.5 and Wing et al.6 It contains the following questions: (i) have you ever heard of the ‘a dead body climbed on top of me’ sensation; (ii) have you ever had the experience of a dead body climbing on top of you; (iii) how many times have you experienced it; (iv) at what age did you have the first episode; (v) during the last month, how many times have you experienced it; and (vi) select the characteristics you have experienced during episode(s).
The ESS is an instrument widely used to assess the subjective perception of sleepiness.14 The ESS Spanish version used in the present study has been found to have acceptable reliability in adolescents (Cronbach's alpha = 0.77).15
Data are presented using frequencies and means ± SD. t-Tests and χ2 tests (or Fisher's exact test) were used for statistical comparisons.
The sample consisted of 322 adolescent subjects (mean age 15.9 ± 0.88 years; 66.8% female).
Of all the subjects, 92.5% (n = 298) had heard about the ‘a dead body climbed on top of me’ expression and 27.6% (n = 89) had experienced the phenomenon. SP (inability to move and/or to speak) was present in 25.5% (n = 82) of the sample while the prevalence rate for hypnagogic/hypnopompic hallucinations (HH) was 22% (n = 71).
The characteristics of the episodes of the ‘a dead body climbed on top of me’ phenomenon are shown in Table 1. In 72% (n = 64) the episodes were composed of both SP and hallucinations while 20.2% (n = 18) consisted of only SP and 7.8% (n = 7) of only HH. These last subjects were excluded from subsequent analyses because they might not correspond to classical SP.
Table 1. Subject data: sleep paralysis episodes
| Inability to move||88.8||96.8||96.0||–‡||1.0|
| Inability to speak||75.3||87.5||78.0||1.1§||0.38|
| Chest oppression||49.4||46.8||54.0||0.39§||0.65|
| Sense of a presence||43.8||37.5||44.0||0.34§||0.64|
| Visual hallucinations||32.6||28.1||36.0||0.54§||0.48|
| Auditory hallucinations||22.5||21.8||22.0||0.00§||1.0|
| Tactile hallucinations||10.1||6.2||14.0||–‡||0.47|
Boys had a higher prevalence rate than girls but the difference was not significant (28.9% vs 23.7%, respectively; χ2 = 1.03, d.f. = 1, P = 0.34). Almost two-thirds of the subjects (61%, n = 50) had two or more episodes in their lifetime and a considerable proportion of them had experienced such episodes several times per year (21.9%, n = 18; Table 2). The mean age of onset was 12.3 ± 3 years (range, 3–17 years; mode, 12).
Table 2. Frequency of sleep paralysis episodes
|Once in a lifetime||32||39.0|
|Two or three times in a lifetime||25||30.5|
|More than three times in a lifetime||5||6.1|
|One per year||2||2.4|
|Several per year||10||12.2|
|One per month||5||6.1|
|One or more per week||2||2.4|
|Almost every night||1||1.2|
The number of symptoms was not significantly different between adolescents with only one episode (n = 32) and those with two or more episodes (n = 50; 3.4 ± 1.4 vs 3.6 ± 1.3, t = −0.64, d.f. = 80, P = 0.52). There were no significant associations between any characteristic of the episodes and the history of one or more than one events (Table 1).
The mean ESS score for the total sample was 8.88 ± 4.0. There were no significant differences in the mean ESS scores either among those with or without SP (9.3 ± 4.4 vs 8.6 ± 3.8, respectively; t = −1.3, d.f. = 320, P = 0.16), nor among those who had experienced it during the last month (n = 23) and those without any recent events (n = 59; 9.0 ± 3.7 vs 9.5 ± 4.7, respectively; t = −0.47, d.f. = 80, P = 0.63).
The present results show that the ‘a dead body climbed on top of me’ expression is highly familiar among Mexican adolescents. This is hardly surprising. In fact, it is consistent with results from studies that have found that 98% of Japanese college students have heard about the kanashibari phenomenon,5 and 93% of Chinese young adults know the ‘ghost oppression’ phenomenon.6
Adolescents were familiar with both the colloquial expression and the experience in itself. Among all participants, one out of four teenagers reported an inability to move or speak, the essential feature of sleep paralysis. As in previous studies, similarities between the characteristics of the phenomenon and SP leave little room for doubt of their equivalence.5,6 There is a small percentage (almost 8%), however, of cases that seem to correspond to only sleep-related hallucinations.
The present results also suggest that SP is a recurrent phenomenon with a high comorbidity with hallucinatory experiences because a high percentage of adolescents had experienced two or more episodes during their lifetime, and in 72% of cases events were characterized by SP with HH. This last finding was not influenced by the frequency of episodes because there were no significant differences in the number of SP features between subjects who had experienced only one episode, and those with two or more events. In this way, as previously suggested by Cheyne,16 it seems to be that folk descriptions capture the motor and hallucinatory nature of SP, in contrast to clinical descriptions, which include only the motor features; episodes of SP alone, however, are less frequent than those comorbid with HH (20% vs 72%). Interestingly, if only those cases of SP alone are considered, we would obtain a lower prevalence rate (5.6%) in the whole sample. Thus, clinical definitions and folk expressions might be near to the poles of a symptomatic spectrum of SP, with the former being in the monosymptomatic motor extreme and the latter in the comorbid SP/HH end. Therefore, questions based on one motor symptom may inhibit affirmative responses from individuals with polysymptomatic episodes, while folk descriptions might elicit affirmative responses from subjects with solely hallucinatory experiences.
The present study has several limitations. First, data were obtained from a non-probabilistic sample of adolescents belonging to a population that used a colloquial expression to refer to SP. These conditions might limit the generalization of findings, particularly to other age groups. The prevalence rates, however, are close to the range found in studies with young adults,5,6,8 including investigations in populations (e.g. Canadian university students) lacking a common expression for the phenomenon.17 Second, data were obtained through self-report instruments, so that we cannot overlook the fact that some questions might have been misunderstood. Finally, a memory effect might still be present even when the period of time elapsed between an episode of SP and the collection of data was short.
In summary the present findings suggest that (i) the ‘a dead body climbed on top of me’ phenomenon is a common experience among Mexican adolescents and it seems to be equivalent to SP; and (ii) during adolescence, SP is a recurrent phenomenon frequently accompanied by hallucinatory experiences.
We are grateful to Paola Jiménez-Genchi and María de la Paz Rojas for their assistance during the application of the instruments. We also thank the students and their parents for their generous participation.