Twenty-five years later, Shelley Adler's new book pieces together what happened, drawing on interviews with the Hmong population and analyzing the extant scientific literature. Sleep Paralysis: Night-mares, Nocebos, and the Mind Body Connection is a mind-bending exploration of how what you believe interacts with how your body works. Adler, a professor at the University of California, San Francisco, comes to a stunning conclusion: In a sense, the Hmong were killed by their beliefs in the spirit world, even if the mechanism of their deaths was likely an obscure genetic cardiac arrhythmia that is prevalent in southeast Asia.
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By 1986, the Hmong deaths had slowed, but remained a striking epidemiological fact. Adler was a graduate student at UCLA studying traditional belief narratives at the time. She'd been researching what she called "nocturnal pressing spirit attacks," or what scientific literature called sleep paralysis. Fascinatingly, sleep paralysis is known to just about all cultures, and it is almost always associated with nocturnal evil. In Indonesia, it's called digeunton ("pressed on"). In China, it's bei gui ya ("held by a ghost"). The Hungarians know it as boszorkany-nyomas, "witches' pressure." In Newfoundland, the spirit that comes is called the Old Hag, and the experience of sleep paralysis, ag rog, "hag ridden." The Dutch name comes closest to what English speakers know. They call the presence nachtmerrie, the night-mare. The "mare" in question comes from the German mahr or Old Norse mara, which denoted a generally female supernatural being who in Adler's words, "lay on people's chests, suffocating them." The etymology of mare isn't clear, but the term is a fruit of the Indo-European language tree, likely from moros (death), mer (drive out), or mar (to pound, bruise, crush).
Across cultures, night-mare visits play out in very similar ways. Victims experience the strange feeling of being "awake." While they have a realistic perception of their environment, they can't move. Worse, they feel an "overwhelming fear and dread" accompanied by chest pressure and difficulty breathing. Scientists have a pretty good grasp of how all of this happens. The paralysis, the feeling of pressure on the chest, all that is explained quite nicely within the scientific models of sleep. During sleep paralysis, a person experiences an "out of sequence" REM state. In REM sleep, we dream and our minds shut off the physical control of the body; we're supposed to be temporarily paralyzed. But we are not supposed to be conscious in REM sleep. Yet that is precisely what happens during sleep paralysis: it is a mix of brain states that are normally held separate.
And then there is the weird stuff, the Old Hag part, the night-mare. People who have an experience of sleep paralysis tend to feel some horrible, evil being is near them. "I just knew this presence was there. An ominous presence ... not only could I not see it, but I couldn't defend myself, I couldn't do anything," one victim told Adler. This feeling is consistent across cultures, even if it goes by different names and presents through the culture one knows.