What is sleep paralysis?

During REM sleep, when most vivid dreams happen, the brain paralyses the body’s muscles so we do not act out our dreams. In sleep paralysis the mind wakes up before this paralysis switches off, leaving a person conscious but frozen, usually for a few seconds to a couple of minutes. Because the dreaming brain is still partly active, many people sense a presence in the room, pressure on the chest or shadowy figures. A 2011 review found that about 8 percent of the general population has experienced it, rising to about 28 percent of students. It is not dangerous, and it becomes more likely with sleep deprivation, stress, jet lag and sleeping on the back.