Witch-trial records, and other early-modern writings on witchcraft, reveal that in various European societies people complained of being physically oppressed at night by witches and other supernatural beings, the victims of these nocturnal assaults describing a similar set of symptoms. Contemporary English authors termed the experience the "mare" or "nightmare." In the twentieth century, it has been identified as a manifestation of "sleep paralysis." Medical studies and surveys of the condition help us make better sense of the historical accounts, while an awareness of the historical evidence illuminates modern reports of sleep paralysis experiences. 
The historical record shows that experience of bewitchment was multifarious, concerning livestock, goods, chattels, and agricultural processes. However, over the past five centuries the majority of those experiences that were deemed serious enough to lead to the formal accusation, prosecution, or physical assault of supposed witches concerned people suffering from some form of physical or mental disorder. Trying to identify what ailments and bodily experiences people attributed to witchcraft is a speculative task, bearing in mind the sketchy description of symptoms in the records, and the limited diagnostic categories of illness in the past. Yet, behind the descriptions provided by those suffering from supposed witchcraft in early modern and later trial records, a number of modern categories of physical ailment and psychological or neurophysiological condition are recognisable.
This article concerns one such condition, sleep paralysis. Although this has only been properly categorised in the past fifty years, the experience has been a matter of medical discussion for many centuries. In the English language, one manifestation of the sleep paralysis experience was known as the nightmare, and in many European cultures its cause was attributed to witchcraft. This nightmare experience can also be identified in other accounts where people claimed to have been nocturnally oppressed by such supernatural beings as the Devil, animalistic fairies, and spirits of the dead. Combining historical analysis with modern medical knowledge, and comparing modern manifestations of the experience with those described in the historical record, enables us to shed further light on human encounters with the supernatural in both past and present societies.
Sleep paralysis is not rare. Surveys around the world suggest that 20-45% of people experience at least one sleep paralysis episode in their lifetimes (Fukada et al. 1987; Ohaeri et al. 1992; Wing et al. 1994; 1999; Spanos et al. 1995; Blackmore 1996; Cheyne et al. 1999a; Kotorii et al. 2001). The condition is associated with the disturbance of rapid eye movement (REM) sleep, and usually occurs immediately before sleep onset or upon awakening, most often in the early hours. Those affected by sleep paralysis can see and hear, because under REM sleep there is intense central nervous system activity, but they are unable to make significant movements, because muscle activity is suppressed. Likewise, only inarticulate sounds can be made. Most episodes last under ten minutes, although up to thirty minutes has been reported (Thorpy 2001, 6). Sufferers may feel their paralysis has lasted much longer. Figures for sleep paralysis are not representative of nightmare experiences, however (see Cheyne et al. 1999b, 316). The experience here defined as nightmare also has diagnostic features of less frequent occurrence, perhaps among 5-20% of the population.
With the nightmare, sleep paralysis is accompanied by the feeling of a heavy pressure on the chest, choking sensations, and hypnagogic (accompanying falling asleep) and hypnopompic (accompanying waking from sleep) hallucinations. Although these hallucinations usually contain the same fundamental elements, they are shaped by cultural beliefs about the origins of the "attack. …