Sleep paralysis is caused when parts of the rapid eye movement (REM) state of sleep persist or intrude into wakefulness.

This means that you remain temporarily paralysed, but are fully conscious.

REM sleep

Sleep occurs in cycles and each cycle is split into two phases – REM sleep and non-REM sleep.

The brain is very active in REM sleep and most dreams occur during this stage. The body is paralysed, apart from the movement of the eyes and diaphragm (the muscle used in breathing). The paralysis is thought to occur to prevent you acting out the actions in your dreams and potentially causing injury.

Sleep paralysis occurs when the normal muscular paralysis of REM sleep temporarily continues after you've woken up.

Increased risk 

Certain factors make you more likely to get sleep paralysis. They include:

• age – it's more common in teenagers and young adults
• sleep deprivation – sleep paralysis is more common in people who don't get enough sleep
• irregular sleeping patterns – people with irregular working patterns, such as those who work shifts, are more prone to sleep paralysis
• narcolepsy – some people with narcolepsy (where you're unable to stay alert for long periods and sleep is disrupted) may also experience sleep paralysis
• family history – you may be more likely to have sleep paralysis if another member of your family also has it; however, further research is needed