Vampires, ghosts and witches are all utterly real – at least to those who witness these horrifying apparitions, explains Roger Highfield.
In pagan times, the last evening of October was "old-year's night", when disembodied ghouls and spirits staged a carnival, and bonfires were set on hilltops to scare them away.
'Vampire legends are quite widespread, although current Western ideas about vampires appear to have originated in a kind of vampire-craze in the 18th century'
The souls of the dead revisited their former homes, while ghosts, witches, hobgoblins and demons roamed far and wide. Today, science can shed new light on the darkest horrors of Hallowe'en.
The legends and superstitions are remarkable because they are not fantasies - the latest research shows that the terrifying creatures that populate the annual celebrations are utterly real to those who see them.
They are the bizarre products of flawed and faulty neurological processes, holding up a distorted mirror that allows us to glimpse the stranger recesses of the human mind.
Take vampires, ghosts and witches. One clue to their origins comes from studies of sleep paralysis, a penumbra of consciousness when sufferers sense the presence of a nearby threat, either in the process of falling asleep or awakening.
Some sufferers hear indistinct voices and demonic gibberish, while others experience hallucinations of humans, animals and supernatural creatures. The condition gets its name because a common element is a striking inability to move or to speak, or the sensation of a weight on the chest.
This is because the brain paralyses the sleeping body to stop us acting out our dreams. Not surprisingly, these bizarre experiences are accompanied by terror.
Recent studies suggest that sleep paralysis may strike about 30 per cent of us at least once. One expert, Allan Cheyne, of the University of Waterloo in Canada, has studied around 40,000 cases from around the world.
He finds that there are often reports of a "sensed presence", such as a spectral figure or ghost, or even an elderly person dressed in an old-fashioned costume. This phenomenon, Professor Cheyne believes, is as old as humanity.
In Newfoundland, sleep paralysis is called "old hag", because it is linked with visions of an ugly old woman squatting on the chest of a paralysed sleeper.
The Chinese refer to it as "gui ya", or "ghost pressure", while in the West Indies there was "kokma", when a ghost baby bounced on the sleeper's chest and attacked the throat.
In ancient Japan, a giant devil was blamed. In fact, sleep paralysis probably gave us the term "nightmare".
The monsters we now associate with Hallowe'en arose, then, because people made sense of this experience by drawing on what seemed plausible in the culture of the time. Hundreds of years ago, witches, demons and ghosts got the blame. In the 1980s and 1990s, people were more likely to report alien abductions.
"Aliens have become merely the latest actors in the ancient drama of the sleep paralysis nightmare," says Prof Cheyne, though he adds that most sufferers "conjure up rather ancient and traditional demons and ghosts".
But how about the vampire - the most familiar night predator of all, condemned to rise from the grave to feast on the blood of sleeping victims?
"Vampire legends are quite widespread, although current Western ideas about vampires appear to have originated in a kind of vampire-craze in the 18th century," says Prof Cheyne.
"In contrast to the suave and urbane vampires of fiction portrayed by Bram Stoker, however, vampires of legend are typically described as repulsive, bloated, and unshaven, with long nails, and tattered funereal clothing." His survey of sleep paralysis revealed hundreds of cases of such vampire imagery, "though a small percentage of the total".
In 1998, the Spanish neurologist Juan Gomez-Alonso linked the Dracula legend to rabies, caused when a virus invades the nervous system and inflames the brain. Symptoms include insomnia, an aversion to water, mirrors and strong smells (though not specifically to garlic), and an increased sex drive.
And rabies, of course, is transmitted by bat and other animal bites, reflecting how the victims of vampires usually become vampires themselves.
"This plausibly accounts for things associated with the 17th- and 18th-century scares," says Dr Tim Taylor, an archaeologist from the University of Bradford who appears in a Hallowe'en documentary, Real Vampires (Discovery Channel, 9pm tomorrow). "The light-sensitivity and fear of water lead to shunning the village priest, with his shiny crucifix and holy water."
Dr Taylor has come up with an even more intriguing idea to explain why we remain fascinated with vampires: it is hard-wired into our brains because our primate ancestors used to show their fangs to demonstrate their status and say: "I am an alpha male and I want sex!"
Gorillas and chimpanzees still do this today, he points out, although humans "now say 'Look at my Jag' instead". Men don't bare their incisors any more because big teeth are, as Dr Taylor says, anatomically incompatible with the evolution of our big brains two million years ago.
But the memory of this evolutionary throwback lingers, not least in the sexier Dracula that prowled around the Hammer Horror films.
Other monsters, such as ghosts and witches, are born in parts of the brain that make sense of what we see.
This has been revealed in studies of people losing their sight who are untouched by the brain diseases, such as Alzheimer's, that are known to cause hallucinations: their visions are conjured up when the brain attempts to make sense of degraded visual information.
These are called Charles Bonnet hallucinations, after the Swiss naturalist who reported his grandfather's strange experiences and later went on to suffer the hallucinations himself.
Dominic Ffytche, of the Institute of Psychiatry in London, found patterns in these visions. Rather than witnessing anything and everything, the patients' apparitions usually fall into a handful of categories, including distorted faces, costumed figures and other bewildering apparitions.
"I'm sure ghosts, fairies and witches all relate in some respect to these disembodied hallucinations," he says. Phantoms were typically small and wore period clothing - 40 per cent saw figures in costume.
"These could be Edwardian costume, knights in armour, military uniforms, Napoleonic uniforms and First World War uniforms," says Ffytche.
"They often wear hats." As for why these apparitions seem to like the same costumes, whether witnessed by patients in India or in Britain, "it is something to do with the brain's visual representation of the human figure - but we do not yet know what".
The disembodied, or distorted face of a stranger with staring eyes and prominent teeth is seen by about half of patients, sometimes only in an outline, like a cartoon. The faces "are often described as being grotesque, or like gargoyles," Ffytche says.
As the brain is starved of sufficient information from an eye that is going blind, it compensates with abnormally increased activity and conjures up hallucinations from the random firing of nerve cells.
His suggestion is that the hallucinations occur when the brain's lateral occipital region alerts us to the possibility that what we are looking at might be a face, an idea backed by brain scans at Yonsei University College of Medicine in South Korea.
This region detects a face's component features - eyes, nose, lips and chin - but does not register where they are. It does not care if a chin is on the forehead, or a pair of eyes under the nose.
Unusual activity in this region seems to make it insensitive to the position of each feature and, says Ffytche, creates "the characteristic distortions of the gargoyle and the over-emphasis of facial features, such as the prominent staring eyes".
People who suffered from Charles Bonnet hallucinations centuries ago bequeathed us the banshees, goblins and crones that stalk Hallowe'en today.
So on a dark Winters night, when you hear things go bump in the night or see shadowy figures, don't be afraid - it's only your mind at work!