Sleep paralysis (SP) is a period of consciously experienced paralysis either when going to sleep or when waking up. It's classified as an REM sleep parasomnia, in other words, an undesirable event that occurs during sleep or that is exacerbated by sleep During an episode of sleep paralysis the individual is fully conscious and aware that it is not possible to move the limbs, head and trunk and there may be also be respiratory difficulties. Indeed, when a person experiences their first episode it's not uncommon for them to think that they are dying, and they may also complain of vivid and terrifying visual, auditory, tactile and/or motor hallucinations.
Sleep paralysis usually occurs whilst lying on a bed and happens less often when a person sleeps in an uncomfortable position, such as sitting upright. Most episodes last a few seconds but may be as long as 10 minutes, and they end suddenly, either spontaneously, because of intense effort to break the paralysis by the person experiencing it, or by the touch or voice of another person. Immediately after an episode there may be numbness or tingling in the hands and feet and there is the possibility of relapse if the individual does not get up and move around.
Prevalence of sleep paralysis Sleep paralysis is a common symptom of narcolepsy which is a sleep disorder affecting approximately 0.02-0.05% of the population. Narcolepsy consists of four major symptoms:
1. Sleep attacks - overwhelming episodes of drowsiness or sleep
2. Cataplexy - sudden loss of muscle tone usually triggered by a strong emotion 3. Sleep paralysis - consciously experienced paralysis whilst falling asleep or waking up 4. Vivid hypnagogic hallucinations - vivid hallucinations at sleep onset.
People who experience narcolepsy do not necessarily experience the symptom of sleep paralysis but in approximately 17-40% of cases sufferers experience vivid hypnagogic hallucinations. Amongst those who also experience sleep paralysis, many will report several occurrences per month, and some say that it happens every time they fall asleep - which can be several times per day.
Sleep paralysis is also experienced by people without narcolepsy, and the prevalence varies, but according to The International Classification of Sleep Disorders (1997) at least 40% of people will experience it once during their lifetime. A further 3-6% will experience it more frequently, and of these, some will experience it severely (episodes occurring at least once per week) and chronically (for 6 months or longer).
Causes of sleep paralysis Sleep paralysis can be thought of as an intrusion of REM (rapid eye movement or 'dream sleep') characteristics into wakefulness. Whilst we sleep the brain suppresses muscle activity, essentially to prevent us from acting out our dreams. But during an episode of sleep paralysis wakefulness occurs yet the body, and part of the brain, is still in REM sleep.
Since REM sleep is usually begins about 90 minutes after we fall asleep, it is not surprising that sleep paralysis is associated with REM sleep when people experience it upon awakening (they have simply woken up during an REM sleep period). But how can it occur when falling asleep? Researchers in Japan have found that people with narcolepsy who experienced the symptom of sleep paralysis often had sleep-onset REM periods (SOREMPs) that is, instead of experiencing REM 90 minutes after sleep onset, they often experienced it immediately upon falling asleep. Furthermore, these researchers found that it was during these SOREMPs that people with narcolepsy experienced sleep paralysis and not during any other stage of sleep.
SOREMPs are also found in people without narcolepsy. They usually occur after disruption of the sleep-wake cycle such as irregular sleep habits, jetlag, shiftwork or sleep deprivation, and these are all considered to be predisposing factors.
Treatment for sleep paralysis Antidepressant drugs that are used to treat the symptoms of cataplexy, sleep paralysis, and hypnagogic hallucinations in narcoleptics typically suppress REM sleep, and it is generally believed that this action of the drug stops sleep paralysis. Some clinicians suggest prescribing antidepressants to people who experience sleep paralysis but do not have narcolepsy if the episodes are frightening, prolonged and frequent.