This article is part of our Villains Week series.

Some of the best fictional villains are inspired by real people and events — Leatherface and Ed Gein, Pennywise and John Wayne Gacy, Biff Tannen and Donald Trump. The interesting part is the way these horrifying realities are reimagined to create make-believe characters that capture pop culture’s imagination.

In the case of A Nightmare On Elm Street’s antagonist, Freddy Krueger (Robert Englund, a sweater-sporting demon with a claw glove that murders teenagers in their nightmares, the horrifying truth that inspired his creation is very odd.

The idea behind Freddy is simple: he’s the deadly manifestation of Somniphobia, otherwise known as the fear of sleep. Anyone who’s ever had a nightmare or experienced sleep paralysis understands that being tucked up in bed isn’t always a pleasant time. We’re also at our most vulnerable during sleep, and thus more susceptible to being attacked. Associating these ideas alone with Freddy makes him scary — but it gets weirder.

In a 2014 interview with Vulture, the film’s writer-director, Wes Craven, explained that Freddy was inspired by a story he read about a young boy who was afraid to go to sleep because he feared death awaited him in his subconscious state. Sadly, he was right.

“I’d read an article in the LA Times about a family who had escaped the Killing Fields in Cambodia and managed to get to the US. Things were fine, and then suddenly the young son was having very disturbing nightmares. He told his parents he was afraid that if he slept, the thing chasing him would get him, so he tried to stay awake for days at a time. When he finally fell asleep, his parents thought this crisis was over. Then they heard screams in the middle of the night. By the time they got to him, he was dead. He died in the middle of a nightmare.”

The example Craven was referring to was one of many of its kind. As noted by Medilink, the medical term is called Sudden Unexplained Nocturnal Death Syndrome (SUNDS), but it has different names in other countries. Regardless of what it’s called, it always means the same thing: nightmare death.

Throughout history, SUNDS has mostly affected Southeast Asian men settling in the United States. There have been several recorded incidents worldwide dating all the way back to 1915, but in the early 1980s they were disturbingly frequent among Hmong immigrants. According to a 1987 report in the Los Angeles Times, at least 130 people had died because of SUNDS, most of them refugees.

As noted in the LA Times article, Dr. Robert Kirschner investigated the phenomenon in the late 1980s, but he could only speculate about what was causing the mystifying occurrences. He proposed that “a random electronic discharge,” which could have been caused by a nightmare, “shorted out” and overloaded their hearts, causing them to stop.

Kirschner also theorized that stress could have been a factor, as most of these deaths occurred within a couple of years of the affected settling in the United States. This reason seems plausible, as settling in a new country often means a struggle to find a job, home, and other necessities that enable a comfortable living.

When news of the deaths spread, it caused so much stress among Southern Asian refugees that they were scared to go to sleep. Many of them believed in spirits and supernatural entities due to their cultural upbringing, so fighting sleep because of the fear of encountering demons wasn’t a far-fetched belief. However, the sleep deprivation may have put so much strain on their bodies that it caused their sudden demise.

Sleep deprivation also increases the risk of sleep paralysis, which explains why people thought they saw demons and monsters in their dreams. In Hmong culture, people believe in nocturnal pressing spirit attacks, and they ward them off with sacrifices and their belief in an ancestral spirit who protects them. By moving to the United States, however, they were no longer able to perform these rituals, and the relocation meant leaving their guardian spirit behind. Thus, they were more prone to evil spirits attacking them.

Fortunately, in the years that have passed since the 1980s, further research has revealed a scientific reason for nightmare deaths. Brugada Syndrome, a genetic disorder in which there is abnormal electrical activity within the heart, is the underlying cause. Research has also discovered that it’s disproportionately linked to individuals of Southeast Asian descent.

Modern cardiology is able to discover and treat heart abnormalities early on, which explains why there haven’t been as many reports of SUNDS since its peak years. That said, while there are plausible scientific reasons for these sudden sleep-related deaths, there’s no denying that for a while, it was an even creepier phenomenon than Freddy himself.