Possible causes of Sleep Paralysis

In surveys from Canada, China, England, Japan and Nigeria, 20% to 60% of individuals reported having experienced sleep paralysis at least once in their lifetime. A study conducted by Sedaghat-Hamedan has investigated the prevalence of sleep paralysis among Iranian medical students. 24.1% of students reported experiencing sleep paralysis at least once in their lifetime. Comparable results were reported among Japanese, Nigerian, Kuwaiti, Sudanese and American students.
Many people who commonly enter sleep paralysis also suffer from narcolepsy
Some reports read that various factors increase the likelihood of both paralysis and hallucinations. These include:

• Sleeping in a face upwards or supine position
• Increased stress
• Sudden environmental or lifestyle changes
• A lucid dream that immediately precedes the episode
• Excessive consumption of alcohol coupled with lack of adequate sleep

In The Terror That Comes in the Night, folklorist and behavioral scientist David J. Hufford argues that sleep paralysis is related to an anomalous experience known in Newfoundland as "the old hag". According to Hufford, the old hag is "an experience with stable contents which is widespread, dramatic, realistic, and bizarre," and elements of the phenomenon cannot be fully explained either by psychology or culture. His works have explored the connection between the old hag and parapsychology in what he labels the "experience-centered approach" to hauntings