*Written by Lark Devon, author of adult fantasy romance where the research shows. Published October 7, 2026.* **Short answer:** Medieval and early modern writers used incubi and succubi to explain nocturnal encounters with sexual and frightening features. Modern sleep research offers a physiological account for some reports: sleep paralysis can combine waking awareness, temporary immobility, chest pressure, and vivid hallucinations of a presence. --- <!-- anecdote? --> Have you ever woken up unable to move, sure something was sitting on your chest, and felt as flustered as you were scared? Medieval writers blamed a demon, and I wanted to know how they got there. I love this one, because the dangerous night visitor is all over the genre I write, and I wanted to see how far back he goes. Christian theologians often treated **incubi** and **succubi** as real demons and asked how a being with no body could touch a human one. The answers got complicated fast, because scholastic theologians chased every metaphysical loophole the question raised. **Sleep paralysis** now gives us a plausible explanation for some of those reports: waking up but stuck, with weight on your chest and something in the room that isn't really there. It can't reach back and diagnose every historical account, though. Follow the idea from medieval theology into gothic art and then into a sleep lab, and you can watch one disturbing experience pick up wildly different explanations along the way. Dark romance still runs on the demon, the pressure, and the dangerous closeness. ## Thomas Aquinas: The Scholastic Mechanics of Demon Seed To see how seriously scholastic writers took the question, I went straight to Thomas Aquinas. In his monumental *Summa Theologiae* (c. 1270), Aquinas tackled a wild theological puzzle in the *Prima Pars* (Question 51, Article 3): can spiritual beings like angels or demons have physical intercourse with humans? His answer ran as a chain of logic: 1. Demons have no bodies, so pure spirits can't naturally produce physical seed. 2. They can condense air into a temporary, tangible body that can be touched and seen. 3. Aquinas reports the claim that a demon can act first as a succubus to receive human seed, then as an incubus to transmit it to a woman. The passage shows scholastic theologians trying to square reported demonic intercourse with the premise that spirits don't generate human seed, and the mechanism they worked out was meant to account for nocturnal sexual experiences and purported demonic conception. ## *Malleus Maleficarum*: The Inquisitors' Obsession with Nighttime Lust By 1487, the church's curiosity about demon anatomy had given way to lethal paranoia. Heinrich Kramer's *Malleus Maleficarum* (*The Hammer of Witches*) worked incubi and succubi into its broader campaign against alleged witchcraft. The traditional co-attribution to James Sprenger is disputed by modern scholarship. The text treats demonic intercourse as malicious and stacks it on top of misogynistic arguments about women's supposed susceptibility to it. That rhetoric helped turn private experiences, rumors, and accusations into evidence inside a system built to persecute the accused. It's tempting to read every account as repressed desire. The historical record won't let me make that call. Sleep experiences, folklore, coercive questioning, theology, and sexual guilt could all have influenced what got reported and what survived. ## Modern Sleep Science: The Neurobiology of Sleep Paralysis So why did these encounters feel so real, so terrifying, and so intensely erotic all at once? In 1999, sleep researcher J. Allen Cheyne and his colleagues published foundational research in *Consciousness and Cognition* on the neurology behind sleep paralysis and hypnagogic hallucinations. The word *incubus* comes straight from the Latin *incubare*, "to lie upon" or "to weigh down." Here's the sequence modern sleep medicine lays out: * During REM sleep, your nervous system suppresses most of your skeletal-muscle movement, called **REM muscle atonia**, so you don't act out your dreams. * If you wake up while your brain is still in REM, your conscious mind switches on before your motor control comes back. You're fully awake and completely unable to move. * Breathing keeps going, but being aware of that stuck-muscle state, and trying to move anyway, can feel like pressure or suffocation on your chest. * Dream imagery and waking awareness can bleed into each other, which can produce a vivid sensed presence. Erotic features show up in some historical demon accounts, but the study doesn't say they're a required part of sleep paralysis. | Element | The Medieval Demonology Theory | Modern Sleep Neuroscience | | :--- | :--- | :--- | | The Intruder | A literal fallen angel or demon taking physical form. | A hypnagogic hallucination caused by REM intrusion. | | The Paralysis | Demonic enchantment holding the victim captive. | Muscle atonia (a neurotransmitter lock on motor neurons). | | The Chest Weight | An incubus sitting physically upon the sleeper's torso. | Restricted voluntary breathing during transitional waking. | | Erotic meaning | Demonic temptation or assault within a theological framework. | A possible feature of the experience or its later interpretation, not a universal symptom. | > [!lore] A living cave turns desire into pressure, helplessness, and a trap Michael and Laura recognize too late. *Sweetest Rot* is available on Lark Devon. [Start reading →](https://larkdevon.com/storytime/story/sweetest-rot) ## Fuseli's *The Nightmare*: From Medieval Terror to Gothic Seduction In 1781, Swiss-British painter Henry Fuseli unveiled *The Nightmare*, one of the most famous paintings to come out of the Romantic era. It's a striking image: a woman in white silk sprawled languidly across a bed, deep in a swooning sleep. An ugly, grinning demon with glowing eyes squats right on her torso, while a ghostly horse with wild, white eyes peers through the velvet curtains. The painting became an influential image right at the border of nightmare, sexuality, and gothic spectacle. * The woman's dramatically exposed pose invites an erotic reading, while the demon and the staring horse keep the whole thing unsettling. * Later viewers have read the incubus through nightmare folklore, sexuality, and the unconscious, and the painting doesn't settle on one of them. Modern dark paranormal romance can recast the incubus as a dangerous lover or an accepting one. Other stories keep the figure's coercive horror. Following the line from Aquinas to Fuseli's demon on a woman's chest, I was struck by how long this mix of fear and desire has stuck around. ## How I Used This in Sweetest Rot <!-- lark-relationship-inline-image -->  Old incubus stories externalized frightening desire as an invading presence. The living cave gives that idea a dark-fantasy form by turning Michael and Laura's already acknowledged desire into a means of control. The comparison shows bodily helplessness and blame while the Cave Dweller remains predatory. The incubus comparison is thematic. The canonical progression remains confidence, ignored warnings, escalating indulgence, and lost control. --- ## What Laura Thinks > *An imagined reaction grounded in Laura's established voice; this is promotional commentary, not an event or quotation from the story.* --- Medieval writers blamed a demon when desire arrived with pressure, fear, and an inability to move. I can't diagnose their bedrooms from a forest cave. I have trouble enough diagnosing the floor. An outside force with a name is still dangerous. Michael and I see warnings and choose to stay before the cave takes that choice away. The old stories and ours ask the same question: when your body reacts under pressure, who is actually in control? If the answer has tentacles, I would prefer to hear it while I can still reach my bow. --- > [!lore] A magical archer, a sword-and-shield warrior, and one warning too many ignored. *Sweetest Rot* is available on Lark Devon. [Read the story.](https://larkdevon.com/storytime/story/sweetest-rot) ## Related Questions ### What were incubi and succubi in medieval demonology? In medieval theology, an incubus was a male demon that visited sleeping women for sexual intercourse, while a succubus was a female demon that visited sleeping men to collect semen. ### What was Thomas Aquinas's theory of demonic reproduction? In *Summa Theologiae* (I, Q. 51, A. 3), Aquinas relayed the claim that a demon could act first as a succubus to receive human seed and later as an incubus to transmit it to a woman. ### How does modern sleep medicine explain incubus visitations? Modern sleep research uses sleep paralysis to explain some experiences of waking awareness with temporary immobility, chest pressure, and hallucinations of a presence. Erotic content is possible, though it is not a required feature. ### What is the origin of the word "nightmare"? The word "nightmare" comes from the Old English *niht* (night) and *mare* (a mythological female goblin or spirit that sat upon the chests of sleeping mortals to cause suffocation). ### How did Henry Fuseli's *The Nightmare* influence romantic literature? Fuseli's 1781 painting became a durable image of nocturnal visitation at the border of terror and sexuality. ## Related Reading - [Why Do We Want the Villain? The Psychology of Morally Gray Love Interests](https://larkdevon.com/blog/why-do-we-want-the-villain-the-psychology-of-morally-gray-love-interests) - [Why Do Women Read Monster Romance? Researchers Actually Asked](https://larkdevon.com/blog/why-do-women-read-monster-romance-researchers-actually-asked) - [Why Are Vampires Sexy? They Weren't Always](https://larkdevon.com/blog/why-are-vampires-sexy-they-werent-always) ## Sources 1. Aquinas, Thomas (c. 1270). *Summa Theologiae*, Prima Pars, Question 51, Article 3. Fathers of the English Dominican Province edition (1920). https://www.newadvent.org/summa/1051.htm#article3 2. Kramer, Heinrich (1487). *Malleus Maleficarum* (*The Hammer of Witches*). Traditionally co-attributed to James Sprenger. Translated by Montague Summers. Dover Publications (1971). ISBN: 978-0486228020. 3. Cheyne, J. A., Rueffer, S. D., & Newby-Clark, I. R. (1999). Hypnagogic and hypnopompic hallucinations during sleep paralysis: Neurological and cultural construction of nightmares. *Consciousness and Cognition*, 8(3), 319–337. https://doi.org/10.1006/ccog.1999.0404 4. Freud, Sigmund (1900). *The Interpretation of Dreams*. Oxford World's Classics edition translated by Joyce Crick (1999). ISBN: 978-0199537587. 5. Davies, Owen (2003). The nightmare experience, sleep paralysis, and witchcraft accusations. *Folklore*, 114(2), 181–203. https://doi.org/10.1080/0015587032000104211