Model and reality TV star Kendall Jenner had previously spoken about her 'sleep paralysis' experience where a person is temporarily unable to move or speak when he/she wakes up or falls asleep.
Though the experience lasts only for a few seconds, it is often very frightening. A recent research shows that the condition has a much wider number of factors than previously thought.
"Most patients say the same thing to describe sleep paralysis: that it feels like you woke up dead. You know that your mind is awake and your body is not — so you're trapped, essentially," Michael Breus, a clinical psychologist and fellow of the American Academy of Sleep Medicine, told BuzzFeed.
According to Daily Mail, in the recent study, experts have identified stress and post-traumatic stress disorder (PTSD) as the main reasons for the condition. Researchers at the Goldsmiths University of London came to the conclusion by reviewing 42 studies.
Though that's not the only reasons, they also say that substance use, physical illness, genetics, personality, anxiety, sleep disorders, and psychiatric illnesses can have an effect.
"Sleep paralysis is a relatively common but under-researched phenomenon," wrote the experts from the University in the journal Sleep Medicine Reviews.
They added: "Sleep paralysis appears to be particularly prevalent in post-traumatic stress disorder, and to a less degree, panic disorder."
According to the Handbook of Sleep Medicine, between 8 percent and 50 percent of people experience sleep paralysis at some time, and about 5 percent of people have regular episodes.
In the review, the researchers compared the frequency of sleep paralysis episodes with numerous variables such as age, sex, diet, income, ethnicity, genetics, caffeine intake, stress levels and IQ.
The authors mentioned that most of these factors showed negligible or inconclusive links, but it was clear that poor sleep quality was linked to sleep paralysis attacks.
Also, patients diagnosed with PTSD was seen to have had significantly higher rates of sleep paralysis. The authors revealed that further research is needed to find out which comes first -- sleep problems or anxiety.