Sleep paralysis is a broad term used to describe a variety of symptoms, but most closely associated with the experience of waking from sleep (or waking at the point of entering sleep) experiencing paralysis of the body. This can be accompanied by hallucinations, feelings of fear and a crushing pressure on the chest.
In normal sleep, a sleeper’s body and brain activity will slow down as he or she falls into deep sleep, before speeding up again as the sleeper enters rapid eye movement (REM) sleep. It is in REM sleep that most vivid dreaming takes place. This cycle between sleep stages takes 1.5 – 2 hours and repeats several times throughout a night (with cycles increasingly weighted towards REM sleep).
When entering REM sleep, the brain shuts down the release of certain neurotransmitters in order to induce a state of paralysis. The body cannot move, and so the sleeper cannot act out the activity of their dream-life (which would endanger themselves and others). In normal sleep, this paralysis ceases before the sleeper becomes consciously awake. Sometimes, however, the process falls out of step. A person may enter a state of waking conscious and become aware of their body while the body itself is still paralysed. In some cases, the combination of wakeful and dream consciousness can cause frightening and convincing hallucinations. It is worth noting that while sleep paralysis is one common symptom of narcolepsy, the majority of sufferers are not narcoleptic.
For a fascinating introduction to the basic causes and cultural interpretations of sleep paralysis, watch Prof Christopher French’s lecture: Something Wicked This Way Comes: Causes and Interpretations of Sleep Paralysis (below). Prof French is Head of the Anomalistic Psychology Research Unit at Goldsmiths, University of London.