This disorder is characterized by temporary paralysis of the body just after the patient wakes up (medically known as hypnopompic paralysis) and just after the patient falls asleep.
When a person is in the REM stage of sleep, his or her skeletal muscles are paralyzed. This condition is medically known as REM atonia. If the sleeper’s brain comes out of REM before the body, the person can become conscious while still having paralyzed muscles. Microarousals are common between sleep stages. The combination of a dream during REM and the odd sensation of being paralyzed while awake can cause subjects to report surrealist conditions. People report seeing ghosts or feeling someone sit on their chest. The phenomenon known as lucid dream can include this state, although it also may include other states of consciousness.
Another hypothesis is that the brain does not truly awaken during these microarousals in which sleep paralysis is experienced, but only part of the brain is awake. Other parts of the brain continue to dream, which the waking part of the brain sees as hallucinations.
Sleep paralysis attacks typically happen in the first two hours of sleep, during or around the first REM stage of the night. It typically lasts from a few seconds to two minutes.
Doctors have identified certain factors that may aggravate the chances of sleep paralysis. They include:
But this sleep disorder is understudied, compared to many others, and risk and causal factors are not fully characterized.
Almost everyone suffers at least once or twice from sleep paralysis during their lifetime. Some people have it more often, and although it can seem creepy to the sleeper, it is not medically dangerous.
How common is this disorder? Estimates of the prevalence of sleep paralysis are all over the place; studies done in different countries produce vastly different numbers. This is almost certainly due to how the researchers ask the question and what the subjects perceive as abnormal. Superstitions and believe in supernatural forces may play a part. A meta-analysis by researchers at Penn State University found "7.6% of the general population, 28.3% of students, and 31.9% of psychiatric patients experienced at least one episode of sleep paralysis" A study in Japan found 8.3% of teenagers self-reported incidence of sleep paralysis. A poll of Irish university students showed about 20% of respondents got the paralysis.
Doctors have prescribed the benzodiazepine Clonazepam – commonly used for patients with seizures – to help relieve sleep paralysis symptoms in severe cases.