Frequency and Features of Sleep Paralysis Among Medical Students of Allama Iqbal Medical College, Lahore

Introduction: Sleep paralysis (SP) is a common condition occurring at sleep onset or offset. Hallucinations are commonly experienced during this phenomenon. An association of stress, anxiety and depression has also been found with the occurrence of sleep paralysis. Aims & Objectives: To observe the frequency of sleep paralysis in medical students of AIMC, Lahore and its associated features. Place and duration of study: This cross-sectional study was conducted at Allama Iqbal Medical College, Lahore from October, 2017 to December, 2017. Material & Methods: 250 medical students participated in the study. A structured questionnaire was made using data from Waterloo Unusual Sleep Experiences Questionnaire. Data was analyzed using IBM SPSS 20.0. Results: Out of 250 students, 89 (35.6%) experienced sleep paralysis while 161 (64.4%) had never experienced it. The number of males who had sleep paralysis was 33 out of 86 (38.4%), while 56 out of 164 (34.1%) females had experienced SP. Only 11 (12.4%) had one lifetime experience while the rest 78 (87.6%) had multiple episodes. The association with anxiety, stress or depression of SP was found in 29 out of 89 students (32.6%) who had sleep paralysis. Conclusion: The frequency of sleep paralysis is about one third in medical students and significant number of students suffered from associated features.


INTRODUCTION
Sleep paralysis can be defined as a period of inability to perform voluntary movements at onset of sleep or upon awakening and is characterized by intense cortical activation and lack of skeletal muscle tone 1,2,3 . This is usually associated with fear, anxiety and may also embrace hallucinations (visual, auditory or tactile), feelings of chest tightening or difficult breathing 4,6 . Respiratory and ocular movements, can usually be performed 5 . It is terminated spontaneously or by an external stimulus and usually lasts for few seconds to several minutes 7,8 . Sleep paralysis is often linked with other sleep disorders like narcolepsy 9 . When outside the diagnosis of narcolepsy, it is called "Isolated Sleep Paralysis (ISP)" 10 . If isolated sleep paralysis occurs frequently, which is rare, it is called "Recurrent Isolated Sleep Paralysis (RISP)" 11 . Associated factors of sleep paralysis include obstructive sleep apnea, depression, panic disorder, post-traumatic stress disorder, stress and hypertension 12 . According to many reports it occurs mostly in supine sleeping position 13 . The hallucinations experienced during SP are of 3 types; intruder hallucinations (sense of presence of someone), incubus hallucinations (breathing related problems e.g. pain and pressure on chest) and vestibular/motor experiences (out of body experiences e.g. floating); the former two frighten the subject, while the latter is associated with blissful feelings 14 . There is no proven treatment for isolated sleep paralysis.
However, drugs like tricyclic antidepressants, serotonin agents and melatonin are used 10 . The condition is not generally dangerous but is associated with panic attacks 25 . Not many efforts had been made previously to study sleep paralysis among medical students. We conducted this study to determine the prevalence of sleep paralysis and associated factors related to sleep paralysis among medical students of Allama Iqbal Medical College, Lahore.

MATERIAL AND METHODS
A cross-sectional study was conducted at Allama Iqbal Medical College, Lahore which is affiliated with Jinnah Hospital Lahore from October 1 st , 2017 to December 30 th , 2017. Consent for research was taken from Community Medicine Department of Allama Iqbal Medical College, Lahore. 250 medical students participated in the study. A structured questionnaire was made using data from Waterloo Unusual Sleep Experiences Questionnaire. The data was collected using non-probability/purposive sampling.

Statistical analysis
Data was entered and analyzed in IBM SPSS Statistics 20.0. Mean and standard deviation were calculated for numerical variables like age. Frequency and percentages were calculated for nominal variables like symptoms of sleep paralysis. Chi-square test was used with p < 0.05 as statistical significance.

RESULTS
Out of 250 participants, 86 (34%) were males and 164 (66%) were females. The age of respondents ranged from 20-24 years. Results showed about 89 (35.6%) students have experienced sleep paralysis at least once in their lives. The number of male students who had experienced sleep paralysis was 33 out of 86 (38.4%), while 56 out of 164 (34.1%) female students had experienced SP. Of all those who experienced it, 11 (12.4%) had only one episode, 45 (50.6%) had 2-4 episodes, 19 (21.3%) had 5-20 episodes while 14 (15.7%) had more than 20 episodes in their lifetime (Fig-1). Out of 250 students, 133 (53%) were boarders (living in hostels) and 117 (47%) lived at home or with a relative. No significant association was found between the residence of respondents and the occurrence of sleep paralysis (x 2 =0.13, df=1, p>0.05). The age when they first experienced this phenomenon ranged from 9-22 years with a mean age of 16 years. Position of sleeping when they experienced sleep paralysis is given in (Fig-2). Half of those who experienced sleep paralysis also felt the presence of someone. Almost 53 (60%) felt pressure on their chest or bodies and the same percentage had a feeling of being suffocated, smothered or strangled, 40 (45%) experienced tactile hallucinations, 27 (30.34%) experienced auditory hallucinations and 58 (34.83%) had vestibular/motor sensations. (Table-1) The association with anxiety, stress or depression of sleep paralysis was 29 (32.6%) while the rest said these factors had no association with the experience. When asked about the cause of this event, 24 (27%) said it was psychosocial, 26 (29%) thought it was physiological, 8 (9%) believed there was a supernatural cause and 31 (35%) had no idea about the cause. Only 43% had experiences that lasted for 1-2 minutes; while for 57%, it lasted for a few seconds.

DISCUSSION
Our investigation revealed, 35.6% of the medical students had at least one lifetime episode of sleep paralysis which is higher than the 28% a previous study found in students (non-medical) 24 .A study in Nigerian nursing students indicated that SP was slightly more prevalent among the male population as compared to females 18 . This is in accordance with our results with male students having predominance over female students. Spanos and colleagues found that ISP is more common than RISP in undergraduate college students 23 . This was not consistent with our findings, according to which only 12.5% had one lifetime experience while the rest 87.5% had multiple episodes. In a previous study, the average duration of ISP episodes was found to be 3.59 minutes 10 . In our study, more than half had experiences that lasted for a few seconds and about 43% said that episode lasted for 1-2 minutes. Although the study on medical students in Nigeria also reported 41.86% of students had experience that lasted about one minute, 39.53% subjects claimed it lasted two to five minutes. Generally, the subjects first experience this phenomenon in adolescence 22 while our study revealed that the mean age of onset was 18 years. Our results indicated that majority of participants experience SP in the middle of sleep while Cheyne in 2002 found out that Sleep paralysis frequencies were higher in the beginning and middle of sleep, as opposed to the end of sleep. The study also stated that 58% of sleep paralysis attacks were reported to occur in the supine position (lying on back), much higher than any other position. This was found in our study too where more than half of the respondents had this experience while lying in supine position 19 .
While studying different types of hallucinations associated with SP, vestibular/ motor hallucinations were most commonly reported, followed by intruder hallucinations, while auditory hallucinations were the least common. However, in another survey, intruder hallucinations were most common while visual hallucinations were least 16 . A study among Chinese adolescents reported that sense of weight on chest/difficulty breathing was felt by 43% while in our study, 59% experienced the same 17 . The feeling of suffocation or being strangled during SP was relatively common in our subjects (60%) contrary to another study in which only 20% had these feelings 20 .
The relation of depression, anxiety and stress was also observed in subjects experiencing sleep paralysis. Our study revealed that 29% of the students were suffering from these states while they had an episode of SP. According to a previous survey 13.6% of the respondents had depressive symptoms 12 . Moreover, a great degree of association was found between anxiety and SP in a past research 13 .
When questioned about the cause of the phenomenon, majority of the subjects (35%) had no knowledge about it. Contrary to a previous research in Nigeria that has reported that around 65.11 % of medical students with SP believed the episode was caused by supernatural phenomena, only a few (9%) believed so in our study. 21

Limitations:
This study was conducted on medical students, they are exposed to different stressors comparative to non-medical students and general population that may have led to different frequency and features of sleep paralysis. Due to vast terminology being used to describe SP, the students may have difficulty understanding the questions. The study findings may have been affected by recall bias.

CONCLUSION
Sleep paralysis is a common phenomenon in medical student with male students having greater preponderance. Vestibular/ motor hallucinations were most prevalent. Association with depression, anxiety and stress was found in about one third of medical students who suffered from SP.