What causes sleep paralysis and the terrifying hallucinations that accompany it?

The answer

Sleep paralysis occurs when consciousness returns before REM atonia (muscle paralysis) has lifted, affecting approximately 8% of the general population and up to 28% of students. During REM sleep, the brainstem releases glycine and GABA to paralyze voluntary muscles, preventing dream enactment. When this paralysis persists into wakefulness, the still-active REM dreaming brain generates vivid hypnopompic hallucinations. The classic 'intruder' experience - sensing a threatening presence - results from hyperactivation of the amygdala's threat-detection system during the REM-wake transition. Cultural interpretations vary globally: 'Night Hag' in English folklore, 'Kanashibari' (bound by metal) in Japan, and 'Jinn attacks' in Middle Eastern traditions. Sleep deprivation, irregular schedules, and sleeping supine increase episodes significantly.