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This article was published in the daily newspaper The Sidney Morning Herald, Sydney, Australia, on August 28, 2005.
It's the middle of the night. Suddenly you're aware that you're awake, but you can't move or make a sound.
And then you feel a weight begin to press down on your chest and you feel you can't breathe.
No, you're not receiving a visit by a supernatural evil being and you're not going mad. But you are experiencing what can be a frightening event that is estimated to happen to up to half of the population at least once in their life.
Dr Andrew Ng, director of the Centre for Sleep Disorders and Respiratory Failure at St George Hospital, said sleep paralysis was perhaps the most common sleep disorder.
But, Ng said, despite its prevalence, there had been little research.
During a typical episode, the affected person awoke, and, even though fully conscious, could not move or speak, Ng said.
It could last seconds or even minutes, although it often feels much longer.
About three-quarters of the time, Ng said, it was accompanied by hallucinations that could include a feeling of floating, hearing noises such as people talking, laughing and humming, or even seeing things, such as another person in the room.
As well, said Ng, a feeling of a "threatening presence" was common, and the sense that someone or something was pressing on the person's chest.
Ng said that while sleep paralysis was not completely understood, there were explanations for the often frightening symptoms, which involved some of the physical characteristics of REM (rapid eye movement) sleep being carried on for a short time during wakefulness.
During REM sleep there was extreme muscle weakness, which explained the lack of movement. It was also the time at which vivid and sometimes terrifying dreams occurred, explaining the hallucinations. The sense of an "evil presence" was, Ng said, in part due to a particular section of the brain being activated without reassurance from the other senses, such as sight and hearing, that the person was safe.
The breathing difficulties and sense that someone or something is sitting on your chest could also be explained.
During REM sleep, breathing was often shallow and the chest could not be expanded fully.
Ng said many people did not experience sleep paralysis often.
"Most people don't even realise it," Ng said. "They get really scared. But the next time it happens, they work it out."
Stress, sleep deprivation, disrupted sleep patterns and lying on the back could all increase the likelihood of sleep paralysis.
Shift workers and nurses reported a higher prevalence, Ng said.
And, he said, most episodes occurred in the second half of the night when REM sleep was more prevalent.
Often, he said, many people experienced it as a one-off, but those for whom it continued to cause problems should think about going to see a sleep physician or sleep psychologist.
Dr Brendon Yee, a sleep and respiratory physician with the Royal Prince Alfred Hospital and the Woolcock Institute of Medical Research, said sleep paralysis could sometimes be associated with the sleep disorder narcolepsy.
"You can also get sleep paralysis by itself," he said. "It can be a problem if it's recurrent and with other disorders."
Yee said 1 to 2 per cent of the population might experience the symptoms regularly.
"Multiple sleep paralysis episodes will occur rarely," he said.
"Forty to 50 per cent of people claim to have had at least one episode of sleep paralysis."
Symptoms of sleep paralysis...
