During REM sleep, dream activity ramps up and the voluntary muscles of the body become immobile. This temporary paralysis keeps us from acting out our dreams and hurting ourselves. Sometimes, though, the paralysis persists even after the person wakes up. “You know you’re awake and you want to move,” Kline said. “But you just can’t.”
Even worse, sleep paralysis often coincides with number 7 on our list: hallucinations. In one 1999 study published in the Journal of Sleep Research, 75 percent of college students who’d experienced sleep paralysis reported simultaneous hallucinations. And these hallucinations, when they occur with sleep paralysis, are no picnic; people commonly report sensing an evil presence, along with a feeling of being crushed or choked. That sensation has given sleep paralysis a place in folklore worldwide. Newfoundlanders know it as the “Old Hag.” In China, it’s the “ghost pressing down on you.” And in Mexico, it’s known by the idiom “subirse el muerto,” or “the dead climb on top of you.”
Even today, some researchers suspect that tales of alien abduction may be explained by episodes of sleep paralysis.