If you're a fan of horror movies, you've probably been disappointed at least once when you thought you were in for a good scare, but the film actually fell flat—leaving you laughing at how unrealistic the events were rather than peeking though your hands in terror.

And while fictional serial killers and supernatural storylines can all be scary in their own right, some movies earn a bit more horror cred because they're actually based on real events.

It's easy to make it through a horror flick knowing that it's simply just a movie, but when there is truth behind it, suddenly you find yourself wanting to sleep with the lights on. Once you know to true story behind the events in classic horror films, you'll never be able to watch them the same way again. If it could happen to these people, it's possible it could happen to anyone.

Here are the real-life inspirations behind some classic horror movies.

The Exorcist (1973)

Director: William Friedkin

Stars: Ellen Burstyn, Max von Sydow, Linda Blair

While The Exorcist is one of the most disturbing horror movies ever created, what makes it even more horrifying is it's actually based on a real-life possession of a boy named Roland Doe in 1949. The young boy was close to his aunt Harriet, whose interest in the supernatural caused him to start playing around with a Ouija board. (Side note: We will NEVER use the Ouija board again after learning this!) After his aunt died, the family started to hear strange noises in the walls and room of young Doe, and furniture began to fly across the room.

Doe began suffering from blackouts and would become violent and speak in gibberish. He was then sent to a Jesuit hospital, where he slashed a priest with a coil from his bedspring after an unsuccessful attempt at his first exorcism. The family moved to St. Louis, where they sent their son in a psychiatric hospital. That's when they enlisted the help of two Jesuit priests, Fathers William Bowdern and Walter Halloran, who performed a second exorcism on the boy. According to the records found at the hospital in 1978, words like "Hell" would appear in blood on his skin and he would refer to himself as "Spite." Eventually, the boy yelled out that he was St. Michael and commanded Satan to leave his body. After one last violent fit, he finally returned to his former self.

The movie is based on the 1971 book of the same name written by William Peter Blatty, although he replaced Doe's character with a girl and added events such as her head spinning around and crawling on the ceiling. What's even more unsettling is that the son of one of the actor's in the movie was hospitalized after being hit by a motorbike during filming, which left some feeling like the set was cursed.

The Texas Chain Saw Massacre (1974)

Director: Tobe Hooper

Stars: Marilyn Burns, Edwin Neal, Allen Danziger

The Texas Chain Saw Massacre tops our list of best slasher fans. It follows the story of a group of friends who become victims of a family of cannibals, including the main baddie, Leatherface. If Leatherface chasing innocent souls around with a chain saw wasn't enough to leave you with nightmares, wait till you learn that while the plot is fictional, the antagonist is actually based on the true crimes of Ed Gein (which will be described in the next section).

Psycho (1960)

Director: Alfred Hitchcock

Stars: Anthony Perkins, Janet Leigh, Vera Miles

Interestingly enough, Ed Gein was also the inspiration for Norman Bates in the 1960 classic Psycho. Alfred Hitchcock's film was based on the 1959 novel by Robert Bloch, and later grew to a franchise that includes six films (including a remake), documentary, and TV series that airs on A&E. The plot follows the story of a mentally disturbed man who takes on the personality of his dead mother, killing people who check in to his hotel.

So who was the man that inspired two classic horror films we still love today? Ed Gein was born in Wisconsin in 1906 to an abusive, alcoholic father and extremely religious mother. He wasn't allowed a social life and spent most his time on the family farm doing chores and listening to his mother read the Bible. After his father died, he began rejecting the beliefs his mother instilled in him, and when a fire broke out on the family property, his older brother was found dead, suffering from blunt trauma to the head, although it was ruled as asphyxiation from the fire.

After his mother died, Gein started to lose his sanity, murdering a female hardware store owner (who was found in his house hung upside down and headless), and a local tavern owner, and he would dig up graves, taking those who resembled his mother home with him. Just like in the Texas Chain Saw Massacre, his house was full of human remains, such as mask made from human skin, and he created suits of skin from women so he could pretend to be his mother. Gein spent the rest of his life in a mental institution after being found insane, his farmhouse burned down in an act of arson.

The Amityville Horror (1979)

Director: Stuart Rosenberg

Stars: James Brolin, Margot Kidder, Rod Steiger

This classic haunted house film follows the story of George and Kathy Lutz and their three children, who lasted only 28 days in a Long Island house at 112 Ocean Ave. in Amityville that was said to be under the influence of demonic forces. And trust us, they are lucky they made it out alive.

The movie (and 2005 remake) is based on the 1976 book The Amityville Horror: A True Story that told the real-life story about the Lutzes and Ronald DeFeo Jr., who murdered his parents and siblings with a rifle while they slept the house in 1974, a year before the Lutz family moved in. DeFeo testified that he heard voices in the house that convinced him to do the savage act, although he later recanted this statement to blame drugs and alcohol.

The Lutz family began to be terrorized by demonic presences in the house and their personalities began to change, even though they had a priest come in to bless the house (who revealed he heard a voice say "Get out!"). While many events were exaggerated in the movie, some also believe it was all hoax and that the Lutzes had a publishing deal in the works while they were living there. While there are conflicting reports that the house was built on an Indian burial ground, the DeFeo murders really happened, and famed paranormal investigators Lorraine and Ed Warren (who are also involved with other true-story horror movies like The Conjuring and Annabelle) also reported that the house was haunted after experiencing supernatural events and capturing an image of a ghost.

A Nightmare on Elm Street (1984)

Director: Wes Craven

Stars: Heather Langenkamp, Johnny Depp, Robert Englund

A Nightmare on Elm Street is petrifying in its own right because Freddy Kruger doesn't come after you in the real world, he gets you in your sleep. In the slasher film, Kruger (a child murderer who was burned to death) stalks and kills teenagers in their dreams, making them die in reality. Afraid to fall asleep, these teens try to stay up for days, which results in trippy states between the dream and real world. But they can't say awake forever because 1, 2, Freddy's coming for you.

Want to be even more frightened by Freddy? Director and writer Wes Craven revealed that the movie was written based on a real-life story he read in the L.A. Times about a family who escaped the Killing Fields in Cambodia. Once in the U.S., the son began suffering from 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," Craven told Vulture in 2014. "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. Here was a youngster having a vision of a horror that everyone older was denying. That became the central line of Nightmare on Elm Street."

To celebrate the 30th anniversary of the classic slasher film last year, Craven revealed Kruger was also inspired by a Peeping Tom, and his glove was based on the "primal fear of attack by animal claws."

Freddy Krueger's appearance was inspired by a man who stared at me through my window one night. #30YearsofNightmare pic.twitter.com/jxocp6949V

— Wes Craven (@wescraven) October 5, 2014

Now, good luck going to sleep tonight.

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