Sleep paralysis is a temporary state of complete inability to move or speak, occurring just before dropping off to sleep or just before fully awakening from sleep.

Although sleep paralysis usually lasts for only a few seconds, it can be a frightening experience, and often it's accompanied by sensory hallucinations or a feeling that someone or something else is present.

Sleep paralysis symptoms are similar to reports from people claiming to have awakened after having

• been abducted by aliens
• seen a ghost in the room
• levitated from the bed while still in a prone position
• felt a malevolent presence (incubus or succubus) sitting or lying on them
• seen objects moving around the room of their own accord
• heard voices or singing
• experienced other paranormal or supernatural events.

In sleep paralysis, the dream imagery and muscle paralysis of the REM sleep state intrude into the waking state.

The waking brain then becomes confused about what's real and what isn't - and being unable to move or speak makes the disturbing real dream images more terrifying.

Approximately 25% of the population has had at least one sleep paralysis episode.