Some people have nightmares of being crushed to death, either by a person or a thing. In the Middle Ages this type of dream was so common that had it a name: incubus (which means ‘the crusher’ in Latin).

In his article ‘Fear, Fantasy and Sleep in Medieval Medicine’ William MacLehose looks at what medieval people wrote about the incubus, which some believed was the work of a demon, while others saw as a medical disease. He writes that “the incubus is an exceptional category as understood in medical discourse, in that it straddles the divides between learned and popular, theological and medical, religious and secular. It also provides a very important example of the interplay of the psychological and mental states, as understood in medieval medicine.”


Arnald of Villanova, a thirteenth-century physician, offers the most in-depth account of what was happening to a person:

They know that they cannot speak nor freely make a sound, they attempt to remove the perceived impediment, for they strive to rise up, call out and speak, and because they cannot, they exert themselves but find themselves constrained. They perceive the cause of the impediment […]. With their spirits bound in a light sleep [and] because their imagination and estimative faculty have not yet been bound by that sleep, since it is light and has just begun, they observe in a shape preserved in the repository of the imagination the shape of that thing which most closely signifies the cause of the impediment. Thus, since the chest and throat are prevented from expanding, they see through their imagination the shape of someone lying upon and strangling them.

The medieval medical writers all note that the patients believe that some sort of creature was attacking them at night. Bernard de Gordon notes that “the common people say that it is little old woman stomping and pressing on their bodies,” while other writers found that it can take many forms. In some cases the victim even finds themselves being sexually assaulted. In general they believe that it happened because when a person was sleeping, their ability to reason stops but their ability of imagination continues. This leaves them susceptible to an attack of incubus.


The medieval physicians offer various explanations on what can trigger incubus – one found that eating raw foods before going to sleep could lead to vapour of “fatty smokiness” would rise up within the body and block the heart and brain, thus inducing the attack. Others found that the position of the sleeper, how warm the bedroom was, or even an overabundance of phlegm could result in incubus. Many writers connected it with epilepsy, and even find it to be a minor form of that disease.

While medieval medical sources condemn the idea that this was some kind of demonic attack, MacLehose finds that other accounts show that some people really believed that this was the case. In the late twelfth-century a knight named Stephen of Hoyland complained that for thirty years he was being attacked at night by a demon who would try to crush or suffocate him. Stephen would even have his servants nearby when he slept, so they could wake him during such an attack, even by “violently shaking him by his hair.” All medical treatments fail, but Stephen prays to Thomas Becket, and the saint miraculous defeats the demon, ending the knight’s torment.

The medical writers offers their own treatments, which often really making the patient feel less stressed. Bernard de Gordon states that “he should live in joy and happiness and avoid all sadness,” and suggests making the patient feel better by having music played or conversations with friends. However, if a person is being attacked by incubus, then you “should have a beloved friend who, upon hearing him calling out and almost lamenting, should wake him” and then try various ointments or medicines.


The article ‘Fear, Fantasy and Sleep in Medieval Medicine’ can be found in the book Emotions and Health, 1200-1700, edited by Elena Carrera. Published by Brill in 2013, it offers eight articles that looks at psychological and mental issues in the medieval and early modern worlds. William MacLehose is a Lecturer of Medieval Science and Medicine at University College London. Click here to visit his page on the UCL website.

See also: Medieval Dreams: A Sample of Historical and Psychological Criticism

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