Why waking up paralysed so often comes with a figure in the room
You wake up, fully aware, but cannot move or speak. A dark shape seems to stand by the bed, or something presses on your chest. Somewhere between 8% and 50% of people go through sleep paralysis at least once, and it may lie behind many tales of alien abduction and other paranormal visits.
Episodes happen while falling asleep or waking and usually last a few minutes, commonly one to six. The paralysis is not total: recordings show the eyes can still move, though speech is impossible. People report hearing humming, hissing, buzzing or voices, feeling pressure on the chest, sensations of sinking, being dragged or flying, and electric tingles. About 5% of people have regular episodes, and men and women are affected equally.
The leading explanation is a faulty overlap between REM sleep, when the body's muscles are normally switched off, and waking. People prone to it tend to reach REM unusually quickly and have fragmented REM cycles, which fits the observation that disrupted sleep can set off an episode. Another theory points to an imbalance between brain cell groups that keep us asleep and those that rouse us, so the sleeping state lingers after the mind has woken. Identical twins often share the tendency, suggesting a genetic component. Risk rises with sleep deprivation, erratic schedules, stress and, notably, sleeping on one's back.
The hallucinations fall into three linked types: an intruder, a sensed presence, and floating or out-of-body sensations. The first two relate to a threat-detection system in the brain. Waking helpless and immobile triggers an emergency response far stronger than in ordinary dreams, which may be why the visions feel so vivid. When the sleeper tries to breathe deeply and cannot, the brain may interpret that resistance as a being sitting on the chest, and mounting fear feeds the loop.
A brief spike of activity in the cerebellum, which tracks body position, may produce the floating feeling. It can be hard to tell apart from narcolepsy-related attacks, which usually strike when falling asleep; isolated episodes more often come on waking.
Source: Sleep paralysis
