Sleep paralysis is a sleep phenomenon and a form of muscular paralysis in which a person wakes up conscious and aware of their surroundings, but unable to move or to speak for a couple of minutes. It is believed (although there are other theories) to involve a dysfunction in the dream phase of sleep when the brain makes the body stay still so one doesn't move when dreaming — and thus reducing the chances of the person getting hurt. If one suddenly wakes up during this phase, for example, they might be relatively conscious, but still temporarily unable to move, in a state between wakefulness and the REM period of sleep; as they still are in a dream-like state, the person may also suffer from hallucinations, panic, breath shortage and sometimes a feeling of pressure or pain in the chest. Because of these factors, many people who experience sleep paralysis report feeling there was a shadowy ghostly intruder in the room with them, either in the corner of the bedroom, in the dark hallway, on the foot of the bed, or on their chest, sometimes suffocating them and making them unable to move or scream for help, as if they had been trapped in their own body. It might lead to lucid dreaming.
With all these characteristics, this phenomenon has been interpreted as being caused by supernatural malevolent spirits — frequently called Night Hags or Old Hags in English — in many different cultures in world folklore since ancient times, often with them possessing the person or sitting on their victim's chest or on the foot of the bed and immobilizing them as they give them nightmares. In Oral Tradition, they can be hags, goblins, demons, imps, ghosts and revenants of dead people, House Fey and overall other supernatural beings and fae-like entities. More recently, they have also been associated with Alien Abductions and encounters. Circumstances can range from a form of communication with the material world, a mischievous prank, or a sadistic and even lethal torment, but the experience is almost always quite unpleasant for the person regardless.
In fact, the English word "nightmare" comes from the Old English "mare" — which in turn comes from the Proto-Germanic "maron" — folkloric goblins or demons who would plague people by causing this experience at night. This holds especially true for the Danish and Norwegian languages, whose words for "nightmare" are respectively "mareridt" and "mareritt", lit. translation: "to be ridden by a mare", which directly invokes the image of the creature sitting on one's chest. A similar process happened with "pesadelo" and "pesadilla" ("nightmare" in Portuguese and Spanish, respectively), which comes from "pesado" (heavy), in reference to the feeling of pressure in the chest during this ocurrance.
Compare Nightmare Weaver, a common role for this kind of folkloric beings, and Things That Go "Bump" in the Night, for other sinister nightmarish monsters lurking around the house at night. Many instances of them don't even have them pose active threats, just putting their helpless prey at unease.
- YuYu Hakusho: In an early chapter when Yusuke is still working to be allowed back into his body, he visits a sleeping Keiko as a spirit to tell her of his plans involving that. While he's not trying to cause any sleep paralysis discomfort, he decides to talk to her while sitting on her belly and making her visibly strain against the weight, invoking this trope.
- The Nightmare by renowned Anglo-Swiss painter Henry Fuseli is an oil painting depicting a woman dressed in all white languishing in bed as a stout demon sits on her stomach, gazing back at the viewer, while a mare stares at the two from behind a curtain. It is one of his most famous pieces that was lauded for its surreal imagery (literally presenting a dream-like image while depicting the contents of one woman's nightmares) and has been interpreted by later art historians as one of the earliest depictions of sleep paralysis as a demon. It has since been the direct and indirect inspiration for subsequent images on the topic.
- In Dead Awake (2016), the social worker Kate Bowman starts to investigate a series of mysterious deaths after her sister dies under unusual circumstances, in which all of the victims died suffering from sleep paralysis. A doctor warns her the killings were made by a malevolent entity, and while Kate at first dismisses it as a superstition, she soon discovers that the entity is haunting her friends and loved ones.
- The Haunting Of Mia Moss: A woman begins to be inflicted with strange nightmares and discovers her mother suffers the same experience when she has sleep paralysis. Eventually, she starts to be plagued by visions of an old hag even when awake and starts to lose her grip on reality.
- Mara (2018) is a supernatural horror film about a criminal psychologist investigating the death of a man who was seemingly strangled in his sleep. When the victim's wife tells her that the man was killed by a sleep demon, she tries to send her to a mental institution. But when the criminal psychologist starts to suffer from sleep paralysis herself and more people keep dying, she wonders if the sleep demon does exist. A doctor later explains the demon is named Mara, creatures from Scandinavian, Slavic, and Germanic folklore.
- In the Swedish movie Marianne 2011, a father coping with the death of his wife begins to be haunted by strange and terrifying nightmares at night. Eventually, he comes to believe his experiences are being caused by a Mare, a being from European folklore who was believed to be the cause of sleep paralysis.
- The Nightmare is a documentary about the phenomenon which features several re-enactments of real-life accounts of sleep paralysis, often with shadowy figures standing in the room with the person.
- The William Castle psychological thriller The Night Walker has a poster showing a satyr-like creature squatting on Barbara Stanwyck's chest as she sleeps. No such creature appears in the movie itself, but the image is clearly meant to be metaphorical for Stanwyck's character being unable to tell where her strange dreams stop and her stranger reality begins. Sleep paralysis, however, is not a theme of the movie.
- In Catalan folklore, the Pesanta is an enormous black dog (sometimes a cat or a creature with no defined appearance) with iron feet that invades people's houses at night through keyholes and door cracks and sits on the person's chest to cause them nightmares and sleep paralysis.
- Mares or Maras are demons in Scandinavian and Germanic folklore thought to sit on people's chests at night and cause nightmares. The beings are also often associated with Succubi and Incubi, and entangled the hair of sleeping beasts, resulting in "marelocks".
- In Portuguese folklore, the Fradinho da Mão Furada (little friar with pierced hands) is a mischievous household sprite who can either help or make pranks and trickery in the house. They can enter the house at night through the keyhole and sit on a person's chest to cause this experience and give them nightmares.
- In Turkish folklore, the word karabasannote is used for both sleep paralysis itself, and the associated creature. It is usually described as a shadowy, sometimes cloaked humanoid figure akin to Shadow People. However, there are no set descriptions about the karabasan. Depending on local customs or beliefs, they may be interpreted as spirits, genies or demons. They can also be seen as bad omens or a manifestation of something wrong in one's life. Regardless of the exact details, they are universally viewed as malevolent.
- In Iran, they're called "Bakhtak". The appearance varies from region to region, but most of the variations are associated with walnut trees (they're more likely to sit on the chest of someone who's sleeping under a walnut tree than any other place)
- Shadow People are a modern version who frequently appear in accounts on the internet. Most of them take the form of vague black outlines that people usually see during sleep paralysis, though one recurring figure, nicknamed "The Hat Man", wears a hat.
- In The Mysterious Benedict Society books, Mr. Nicholas Benedict has narcolepsy, often causing him to fall asleep at inappropriate moments, usually triggered by strong emotion, often laughter. In the second book, The Mysterious Benedict Society and the Perilous Journey, he reveals that his sleeping fits are often accompanied by terrible nightmares, followed by fits of sleep paralysis, and with these, a feeling of weight on his chest accompanied by a nightmarish creature known as the "Old Hag." He then jokes to the protagonist character, Reynie, that he's shook hands and made peace with her, though what he means to say is that he's grown used to the hallucinations such that they no longer trouble him as they once did. Later on, during a lull in the perilous journey of the book's title, Reynie crashes in a bed in a shack on an island and experiences what he believes to be this, though when he finally rouses himself to full consciousness, it turns out to be Martina Crowe, an Executive of the Big Bad Ledroptha Curtain who has caught up to him.
- H. P. Lovecraft's short story Pickman's Model centres on a collection of spooky paintings by a Mad Artist. The narrator mentions that some of the paintings depict ghoulish creatures "leaping through open windows at night, or squatting on the chests of sleepers", implying this trope, and suggesting the Fuseli painting mentioned above in particular. Earlier in the story, the narrator directly compares the fictional Pickman to Fuseli, suggesting a deliberate invocation on the author's part.
- Guy de Maupassant's short story "The Horla" is about a man who is tormented by an invisible being that sometimes acts this way. He first becomes aware of it when it wakes him by squatting on his chest as he sleeps.
- Grimm: The Alpe feed by sitting on top of people as they sleep, chemically paralysing them and draining their melatonin. They're the in-universe explanation for sleep paralysis and the image in the Grimm diaries strongly resembles Henry Fuseli's 1781 painting, The Nightmare, which depicts the condition.
- The Haunting of Hill House: Throughout her life, Eleanor "Nelly" Crain suffers terrifying episodes of sleep paralysis in which she's haunted by a shadowy figure she calls "The Bent-Neck Lady", most commonly seen standing over her or on the other end of the bed. These episodes number among the worst moments of Nelly's life, and as her depression worsens, the Bent-Neck Lady begins appearing to her even when she's mobile and fully conscious. The twist is that the Bent-Neck Lady is actually the ghost of Nelly herself, scattered across history through the power of Hill House - as Nelly discovers when she's tricked into killing herself and is stuck reliving her worst episodes of sleep paralysis, but this time as her own sleep paralysis demon.
- Brazilian Folklore: The Pisadeira ("she who steps" in Portuguese) is a frail old hag with dirty nails and staring red eyes who lurks on rooftops at night, waiting to step on those who went to sleep with their stomach full.
- Classical Mythology: In Greece and Cyprus, sleep paralysis is caused by a demonic creature called Mora, who sits on its victim's chest, causes breath shortage, and tries to steal their voice.
- Japanese Mythology
- The Kanashibari ("bound to the metal") are the most notable example of this trope, as the literal translation of sleep paralysis in Japanese.
- Many Youkai are also related with this phenomenon.
- Slavic Mythology: The Kikimora is a kind of mischievous female household spirit who, depending on the behavior of the homeowner, can be helpful, helping to clean the house and doing chores, or malicious if her or her work were to be disrespected, taking delight in causing nightmares or even kidnapping children (in contrast to Domovoi, who are benevolent unless mistreated). She, among many other mysterious nocturnal activities, was believed to be the cause of sleep paralysis, immobilizing her victims by sitting on their chests and terrorizing them with thoughts of dying.
- Swahili Mythology: Sleep paralysis is known as jinamizi ("strangled by jinn"), and is attributed to happen as a result of people sleeping on their backs.
- Yoruba Mythology: Ogun Oru is the traditional explanation for many nocturnal disturbances in Southeast Nigeria, including sleep paralysis. It is culturally attributed to demons infiltrating one's body and mind during dreaming, and it is believed to be treated through rituals and Christian prayers.
- The Deep Sleep Trilogy features monsters called Shadow People that haunt your dreams in the first and second games. The third game begins with you waking up paralyzed and seeing a Shadow Person standing in your bedroom, right before it attacks you and sends you back into the dream. At the end of the game, you realize that the Shadow Person possessed your body and trapped you inside the dream forever, turning you into another Shadow Person.
- Don't Open Your Eyes is an indie horror game/interactive story that explores the phenomenon of sleep paralysis. The game starts with a realistic rendering of a bedroom, but when you go to sleep, the style switches to a black and white sketch drawing of the room as you recreate your room from memory. Hearing footsteps however makes you imagine a faceless demon by your bedside who beckons you to look at it. Unable to move, progressing through the story requires you to simply deny this request again and again, while also filling in more details about what this demon looks like.
- God of War (PS4): Zig-Zagged with the Nightmares, floating eye-like enemies with tentacles that attack the player with poison and can possess enemies in-game to make them stronger. They cause nightmares and are inspired by Mares, demons in Germanic and Scandinavian folklore who sat on their victim's chest and caused sleep paralysis. While their appearance differs greatly from their more imp-like mythological counterparts, Atreus' bestiary in his diary suggests they were named after them, who better fit this trope:
"They're named after an older creature, called a Mare, that sits on your chest while you sleep and feeds off your fear."
- Captioning funny, creepy, or weird pictures with "This is my sleep paralysis demon" or similar is a meme. Know Your Meme had an entry on it.
- SCP Foundation, SCP-932 ("Night Feeder"). Normally invisible, they look like featureless, pale children. When a human is asleep and dreaming, a group of them will pin the victim down, wake them up and terrify them to consume their fear.
- Hilda: The Marras are young female human-like spirits who haunt streets and houses at night to give people nightmares, inspired by the Maras from European folklore. This illustration used in a book from Chapter Six resembles more closely the creatures of this trope◊, showing a marra tormenting a man by sitting on his chest.
- Legend Quest: In the episode The Mart, the protagonists head to Germany and face the Nitch Mart, a malevolent creature who had been haunting a city for decades by invading the villagers' houses at night and inflicting nightmares to feed off their fear.