In a landmark 1999 paper, Cheyne, Rueffer and Newby-Clark researched the nature of a sleep paralysis, looking at the correlations between different symptoms. Data was collected from 752 sleep paralysis sufferers who were asked questions about the symptoms they suffered during a sleep paralysis attack. The authors then looked for patterns in the occurrences of these symptoms. They concluded that the symptoms of sleep paralysis fall into three distinct groups:
- Intruder: Sensed presence, fear, auditory and visual hallucinations;
- Incubus: Pressure on chest, breathing difficulties, pain;
- Unusual Bodily Experiences: These may include flying/floating sensations, out of body experiences and feelings of bliss.
In a later study Cheyne examined how the rates of these three different categories of sleep paralysis differed between ‘novice’ sleep paralysis sufferers and ‘experienced’ sleep paralysis sufferers. The main finding was that the ‘novice’ sufferers experienced more ‘intruder’ symptoms and very few symptoms from the ‘unusual bodily experiences’ set. ‘Experienced’ sufferers appeared to suffer less from ‘intruder’ symptoms, and had more ‘unusual bodily experiences’. Incubus symptoms appeared similar across both.
When looking at frequency of sleep paralysis attacks, they found the average age of the ‘novice’ to be lower than that of the ‘experienced’ sufferer. However the most chronic sufferers were not much older than the novices, with older groups falling into the intermediate frequency groups. This suggests that sleep paralysis may be less common in older populations.
Another study on the subject by Cheyne in 2002 looks at situational factors effecting sleep paralysis. It found that the following factors had an effect on sleep paralysis rates:
- Timing: Sleep paralysis frequencies were higher in the beginning and middle of sleep, as opposed to the end of sleep.
- Position: 58% of sleep paralysis attacks were reported to occur in the supine position (lying on back), much higher than any other position (prone = 8%, side = 17%). Sleep paralysis sufferers were found to be no more likely to fall asleep in the supine position compared to non-sleep paralysis sufferers (22% of the total sample indicated falling asleep in the supine position).
Sex, fear & unusual bodily experiences
Attacks occurring at the end of sleep featured fewer intruder hallucinations, as well as less intense incubus hallucinations. The frequency of unusual bodily experiences was higher in attacks at the beginning of sleep, compared with other times. Feelings of bliss and erotic feelings, which are less commonly reported, did not differ in relation to time of occurrence. Despite frequency and intensity of hallucinations being lower at the end of sleep, the highest levels of fear were reported in attacks at this time, with females reporting greater fear than males. In general, females reported more intense incubus hallucinations.