Various risk factors have been identified as associated with sleep paralysis. Sleep disruption is perhaps the most obvious of these, and high rates of sleep paralysis have been reported in those with frequently disrupted sleep patterns such as insomniacs and shift workers. Other risk factors include traumatic life events, anxiety and depression. These conditions have been shown to increase the chances of suffering from sleep paralysis, though the fact that they themselves often cause disrupted sleep patterns indicates that sleep disruption may be the key mediating factor. Below is a summery of some of the research done in this area, with links to the studies in question where possible.
1. Sleep Disruption
A 2002 paper by Takeuchi, Fukuda, Sasaki, Inugami and Murphy provides strong evidence that disruption to the sleep cycle can increase the risk of sleep paralysis. These researchers were able to deliberately elicit sleep paralysis attacks in volunteers by depriving them of REM sleep (waking them up every time they entered this sleep stage). Eventually this caused a sudden onset REM period (SOREMP) where participants bypassed all other sleep stages and went straight into REM sleep from waking. From 184 sleep disruptions, eight episodes of sleep paralysis were recorded.
2. Traumatic life events
In a 2005 paper, Yeung, Xu and Chang looked at rates of sleep paralysis in Chinese and American psychiatric out-patients. They found that panic disorder and post-traumatic stress disorder (PTSD) led to a higher incidence of sleep paralysis, suggesting that these two disorders are risk factors for sleep paralysis. The authors suggest the higher physiological arousal that occurs during PTSD and panic disorder causes disruption to sleep patterns, this increasing the risk of sleep paralysis.
A 2005 study by Hinton, Pich, Chhean, Pollack and McNally looked at rates of sleep paralysis in Cambodian refugees, and found further supporting evidence that suffering traumatic life events can increase the likelihood of experiencing sleep paralysis. Out of 100 refugees, 49 had suffered at least one sleep paralysis attack in the 12 months prior to the study. Those who had been given a clinical diagnosis of PTSD showed higher sleep paralysis prevalences than those who had not been given a diagnosis (65% of the PTSD group suffered at least one sleep paralysis attack per month, compared with just 15% of the non-PTSD group). They also found a positive correlation between severity of PTSD and number of sleep paralysis attacks. This could indicate that while traumatic events may increase risk of sleep paralysis, a sleep paralysis attack can be a traumatic event in itself.
A 2008 paper by Mellman, Aigbogun, Graves, Lawson & Alim looks at rates of sleep paralysis in African Americans. Research indicates that sleep paralysis is more common in African American than white american populations, with some authors pointing to increased social hardship and racism as being contributing factors to this higher incidence rate. Mellman et al found that sleep paralysis was associated with exposure to stress, trauma and depressed feelings but not to any specific psychiatric disorder, other than prior substance abuse. They suggest the role these risk factors play in sleep paralysis is mediated by sleep disruption, as it is known alcohol and substance abuse can severely disrupt REM sleep.
3. Anxiety and depression
A 2006 paper by Otto, Simon, Powers, Hinton, Zalta and Pollack examines rates of sleep paralysis in patients diagnosed with an anxiety disorder. Approximately 20% of the patients suffered from sleep paralysis. The use of medication was not found to significantly affect rates of sleep paralysis. Other research shows that patients with anxiety disorders show severe sleep disturbances, again suggesting a mediating role of sleep distrubance in the increased risk of sleep paralysis in patients with anxiety disorders.
A 2008 study by Solomonova, Nielsen, Stenstrom, Simard, Frantova and Donderi looks at the relationship between the amount of distress caused by a sleep paralysis attack and social anxiety. They found distress levels to be associated with dysfunctional social imagery, including extreme anxiety and fear of death in non-threatening situations as well as feelings of being observed. These factors are all associated with the feeling of a sensed presence in sleep paralysis so it is perhaps not surprising that distress in sleep paralysis was found to be most strongly associated with the sensed presence symptom set.
A 2005 study by Simard and Nielsen also looked into links between sleep paralysis, social anxiety and depression. They found higher levels of social anxiety in participants who suffered from sleep paralysis with sensed presence than those participants who did not sense a presence during sleep paralysis. They suggest someone who is socially anxious in waking life may be more likely to generate the threatening hallucination of an evil presence during dreams in REM sleep, and subsequently in sleep paralysis. They also suggest that the hallucinatory images may replicate or represent prior traumatic social events.
A 2007 paper by Szklo-Coxe, Young, Finn and Mignot investigates depression and its links with a number of sleep disturbances. It finds depression to be strongly associated with sleep paralysis, even after removing factors such as use of antidepressants, excessive daytime sleepiness and insomnia. It also finds that the relationship between sleep paralysis and depression is not explained by anxiety felt in depression, suggesting depression to be a risk factor for sleep paralysis, distinct from anxiety.