The aim of our study was to assess the prevalence of SP and to identify risk factors for its occurrence in a Polish student population. Sleep paralysis remains a poorly researched disorder and work is underway to clarify the risk factors for its occurrence.
The point prevalence of SP in the population of Polish students was 33% which does not differ statistically significantly from the average worldwide prevalence in students, estimated at 28.3% (chi2 = 0,104). To the best of our knowledge, among other European countries, only Ireland has carried out a study on the student population, with a prevalence rate of SP − 19.9% , which is also not statistically significantly lower than in our country (chi2 = 0,5) . The highest prevalence in the student population was recorded in Peru namely 55% . In student populations within other countries, the prevalence of SP varies by study, e.g. in Canada, 29–41.9% of students have experienced at least one episode of SP [56, 57]; in the USA about 25% , in Japan from 38.9–43% [57, 59]; in Nigeria, from 26.2–44.2% [60, 61]; in Egypt 43% , in Kuwait about 29% and in Sudan 30% .
Our assumptions regarding the association of SP with lifestyle factors have been partially confirmed. We found an association between SP and BMI, alcohol consumption, smoking, sleep duration, and physical activity. Subjects who experienced at least 1 SP ever had a higher BMI compared to those who had never experienced it. Similar results were obtained by Sharpless et al, 2010, which showed a positive correlation between the amount of FISP (Fearful Isolated Sleep Paralysis) and the BMI of the subjects . It is interesting that in a 2017 study on the student population by Abdel Wahed and Hassan, BMI over 25 was associated with higher levels of anxiety and stress . The relationship between the incidence of SP and the use of stimulants such as alcohol and cigarettes is not surprising. Previous studies have already confirmed this relationship [19, 24]. Munezawa et al, in their study in a group of 90,081 Japanese teenagers, found a higher incidence of SP among alcohol users (7.1% vs 12.2%) and cigarette smokers (7.8% vs 15.3%) compared to those who did not use these stimulants . It is interesting, however, that in this population of Polish students, the use of these stimulants is associated with a higher odds ratio for SP by as much as 1.52 for alcohol and 1.48 for smoking.
Our observations confirmed that the incidence of SP may be related to the duration of sleep [24, 64, 65]. The mean time of sleep per day was negatively correlated with the number of SP episodes during last year in women. There was a negative correlation in participants who suffered from hematological disorders, asthma, and anxiety disorders. In addition, study participants who slept an average of 5 or fewer hours a night compared to participants who spent an average of 6 to 9.5 hours a night sleeping had more SP during the last month and year. Also, too much sleep ( > = 10 h) was associated with a higher number of SP episodes throughout a lifetime. Munezawa et al., 2011 made similar observations in a group of Japanese teenagers they studied, where the incidence of SP was higher in those who slept fewer than 5 hours or more than 9 hours per day .
The results we obtained regarding the influence of physical activity on the incidence of SP may seem controversial in the light of many studies that document the beneficial effect of physical activity on sleep quality [66, 67]. However, it is important to remember that the effect of exercise on sleep depends on gender, age, BMI, condition, type, and protocol of exercise (intensity, duration, and type of exercise) [67, 68]. Vigorous exercise can significantly affect sleep architecture, increasing the delay in slow wave (SWS) and paradoxical (REM) sleep, reducing the amount of REM sleep [69, 70]. In our study, we did not assess the usual hours of physical exercise and sleep, and as the research suggests, it may be related to the occurrence of sleep disorders, because a negative impact on sleep latency and awakening at night was noticed in people who exercise earlier than 8 hours or less than 4 hours before bedtime [69, 70]. A further investigation extending the research methods with a detailed assessment of the exercise schedule and normal hours of sleep appears worthy of pursuit.
The results of research by Ruotolo et al., 2016, which showed that children involved in physical activity suffered from parasomnia more often (62%) than physically inactive children (38%) is notable . To date, the effect of physical activity on the incidence of SP has not been explored. Our results indicate that there was higher physical activity in participants who experienced at least one episode of SP compared to those who never experienced the phenomenon. Additionally, there is a positive correlation between the number of hours spent on physical activity and the number of SP episodes in people with at least two health problems, mental disorders, or those taking antidepressants which suggests that exercise may be related to SP to some extent. The subject requires a detailed study, bearing in mind that the level of physical activity was not tested by us with special questionnaires, but only with the proprietary sociodemographic survey, which included two questions about sport: "Do you do sports?” and "If so, how many hours a week do you do physical activity?"
As we showed in our study, people with an additional burden of somatic disorders have a higher risk of developing SP and more often experience episodes of SP. Little research has been done to date on the relationship between SP and other diseases. Mume and Ikem, 2009 showed that there are significant differences in the prevalence of SP between healthy and sick people. In their study, SP was reported by 28% of healthy people, 44% of orthopedic patients, and 56% of patients complaining of multiple somatic complaints . In a study by Takeuchi et al., 2002, the group of subjects who experienced isolated SP exhibited poorer performance, more complaints of physical, mental, and neurotic symptoms, and increased subjective fatigue compared to subjects who never experienced SP . Moreover, more frequent occurrence of SP with such disease entities as obstructive sleep apnea and arterial hypertension were also noticed [33, 36].
The correlation between the occurrence of mental disorders and more frequent SP episodes has been confirmed in many studies on sleep paralysis [7, 47, 72]. The results of our study reveal that a high level of anxiety understood as a personality trait (> 44 points) is associated with as much as a 1.20 higher odds ratio for SP. Moreover, participants who experienced at least one episode of SP in their lifetime had significantly higher symptoms of anxiety, which is understood as a personality trait, compared to those who never experienced it. Interestingly, this relationship was observed especially in men. There were no significant differences in anxiety levels among women. In men who experienced at least 1 SP episode in their lifetime, the severity of worry was significantly higher compared to those who had never experienced it. The results of our research suggest that high levels of anxiety as a personality trait and a tendency to worry may be risk factors for developing SP. Jalal and Hinton (2015) also observed such a relationship in the Egyptian student population . The relationship between the presence of anxiety symptoms is also confirmed by the observations of other researchers [13, 17, 34, 73]. Importantly, our research shows that people suffering from mental disorders were not only at significant risk of developing SP but also had significantly more episodes of SP throughout their lives compared to people with a negative history of mental illness. In the SP + group, people suffering from mental disorders (n = 56) were older, had a higher BMI, smoked more cigarettes for a longer period time, had more PTSD symptoms and more incidences of SP throughout their lives compared to mentally healthy people, see Table 2. Interestingly, the results of our study indicate that taking antidepressants is associated with an increased risk of SP. This is surprising, considering that other studies have not confirmed the effect of these drugs on the incidence of SP [8, 18, 21]. Moreover, there are reports of successful treatment of SP with SSRIs and TLPDs [74–76]. On the other hand, it is a fact that some classes of antidepressants may worsen the quality of sleep due to the activation of 5-HT2 serotonergic receptors and increased noradrenergic and dopaminergic neurotransmission. In addition, some drugs from the SSRI, SNRI, and TCA groups also have a negative impact on sleep documented by research . They induce or worsen bruxism during sleep, may induce nightmares, and disrupt muscle tone during REM sleep, which may induce or exacerbate the movement disorder associated with the REM sleep phase . The effect of antidepressants depends on both the class and their dose, so it would be important to carefully study the effect of these drugs on SP. It can be assumed that the more frequent occurrence of SP may be conditioned by many factors, as a component of lifestyle, mental and physical health.
In our study, we did not confirm an association of SP with PTSD what was found in other studies. However, this can be explained by the fact that the student population, despite being exposed to chronic stress, is not at risk of Post-Traumatic Stress Syndrome.
The study has several limitations. The first is the form of an online questionnaire that makes it impossible to clarify the question if the researcher feels it is unclear. The second is that people suffering from SP were more likely to respond to the survey than people who did not experience this phenomenon, which could have influenced the result of the prevalence of SP in the Polish student population. A large sample size may influence the results of a study by showing significant statistical differences even when they are clinically insignificant. Another is that the number of SP risk factors in our online survey was limited. We assume that there are other risk factors influencing the prevalence of SP in the Polish student population. In the future, it would be interesting to extend the research methods to include assessment tools: stress exposure, ways of coping with stress, assessment of depressive symptoms, hygiene and quality of sleep, sleep disorders, physical activity; taking into account the frequency, intensity, and type of training of participants, as well as the usual time of exercise. Another limitation of the study is that women accounted for 80% of the respondents. The reason for this may be that in Poland, in 2018, women dominated at most Universities . Women accounted for 70% of students of humanities and medicine, and almost 70% of those studying art and natural sciences . In the future, it would be worthwhile to conduct a study of faculties focusing on male students. Further research is required to establish SP risk factors in the Polish population.
The results of our research indicate that a large proportion of students experienced isolated sleep paralysis. The variables predisposing to this disorder include both psychological conditions (tendency to worry, high level of anxiety as a constant personality trait), general health (the presence of somatic and mental disorders) as well as behavioral factors (stimulants, sleep time, level of physical activity, medicines). Considering the large scale of the problem, detailed studies would be warranted to further investigate the influence of these variables on the frequency and course of sleep paralysis. Accurate determination of predictors is essential to designing preventive and therapeutic interventions.