A see-through figure floating down a hallway. A creaking floorboard. A looming dark shape in the corner of a room. Stories of ghosts have been around for thousands of years, however, there has been no scientific proof of their existence. So, what causes people to see things that may go bump in the night?

It may shock you to find out that 46 percent of Americans and 34 percent of people in the United Kingdom say they believe ghosts exist, with a startling 18 percent of Americans claiming that they have seen a ghost previously. The reasons for this, it turns out, could be medical.

Poor sleep and sleep paralysis

Imagine you are snug as a bug in a rug, you open your eyes and see a ghostly apparition floating above you ­– you may be suffering from sleep paralysis.

Sleep paralysis happens at a very specific sleep cycle stage. Typically, we start the night with non-rapid eye movement (NREM) sleep. Then we go into rapid eye movement (REM) sleep, and we usually cycle between the two throughout the night. Sleep paralysis happens in the REM stage, when we are having the most vivid dreams, but is also the stage where we are paralyzed.

People who experience sleep paralysis can often feel awake but may have hallucinations and struggle to move. This seems to be the main reason for paranormal accounts, according to Professor Chris French, who is an expert in the field of anomalistic psychology (where people test non-paranormal explanations for paranormal phenomena).

It is not only sleep paralysis, but also poor sleep quality, that can be a factor in people’s belief in ghosts. Research suggests that not getting enough shut-eye at night may contribute to stronger beliefs in ghosts, demons, aliens, and the afterlife. In a survey with 8,853 people questioned about paranormal beliefs and sleep quality, they found that the beliefs are more common in people who struggle with insomnia, report “lower sleep efficiency”, get fewer hours of sleep per night, or take longer to fall asleep.

It has also been found that people who have disturbed sleep can then trigger sleep paralysis, which, as we have discussed above, can involve hallucinations that people may believe are paranormal activity.

Carbon monoxide

Where we live can have a big effect on whether people see ghosts or not. 

Carbon monoxide is a colorless and unscented poisonous gas that can make people seriously ill if breathed in, it can also lead to death. Carbon monoxide can be produced by fires, dodgy furnaces, and other appliances.

A lot of hauntings can be attributed to carbon monoxide poisoning, especially in the Victorian era. In 1921, a detailed report was published in the American Journal of Ophthalmology. It described a curious case of a family who had moved into a house and began to experience “paranormal” activities. The father started feeling that people were watching him from behind whenever he was eating a snack before bed. The children started to become sick, pale, and lost interest in playing. It turns out that the house furnace had broken down and this was a source of carbon monoxide, which made them feel ill and likely explains the “ghostly” activities.

Another more recent case was that of writer Carrie Poppy who felt that there was a demon in the house in which she was staying. When she got in touch with some paranormal skeptics, they asked her if she had heard of carbon monoxide poisoning as the symptoms lined up with what she was experiencing (feeling of dread, pressure on the chest, auditory hallucinations). She believed that this was the reason.

When she got a gas company out to investigate, they confirmed that she had a carbon monoxide leak. If she had left it any later, she would have died shortly after.  

Mold

There is also the possibility that mold can cause some ghostly sightings. There was a team at New York’s Clarkson University that investigated the link between haunted houses and mold. The team was able to collect data from 27 locations (13 considered “haunted”) and found that there was a statistically significant link between haunted locations and mold.

Suggestibility

There was one research team that attempted to build a “haunted” room. They did this by trying to manipulate the environment by engineering the electromagnetic fields and infrasound. The 79 participants had to spend 50 minutes in a specially constructed chamber. Although many reported various kinds of anomalous sensations, it was unrelated to the experimental conditions, which indicates that suggestibility was the biggest factor.

So, if you are told a room or house is haunted you are more likely to think you experience the paranormal.

Brain anomalies

Sometimes the brain can make you see weird things. One man was being experimentally treated for epilepsy by electrode strips being implanted in his fusiform gyri – this region plays a key role in visual and pattern recognition.

Researchers sent electrical pulses into the fusiform face area and the patient began to see illusory faces all over the place, including within the real face of a person. These were hallucinations that were referred to as facephenes by the science team, however, only certain people may see this.

Other researchers did experiments to induce the creepy feeling that somebody is close by. They did this by stimulating a site in the brain’s left hemisphere. The 22-year-old patient was undergoing evaluation for epilepsy treatment and when parts of her brain were stimulated she felt like someone was behind her, describing the “person” as a “shadow” who did not move or speak, directly behind, and was young and of indeterminate sex.  

This is not the first time this was done. There was a robot that also made people feel like there was a ghost behind them. In work produced in 2014, the robot could make healthy participants feel like there was someone (or something) standing behind them, due to the spatiotemporal conflict they were experiencing.  

Following on from this research, neuroscientist Olaf Blanke decided to look into people with neurological conditions, as there are some diseases that can cause people to see ghostly figures. Around 50 percent of people who have Parkinson’s disease (a disease that causes parts of the brain to become more and more damaged over the years) report that they experience hallucinations. They affect both women and men, and the hallucinations can vary from seeing things out of the corner of someone’s eye to fully formed and colored visual hallucinations.

Using an improved version of the same robot, the team tried to induce presence hallucinations in Parkinson’s patients. When the team stimulated different areas of the brain, the patients felt a presence or “ghost” behind them. It was found that those patients with pre-existing presence hallucinations had a heightened sensitivity than those without any hallucination experience. This research means that a disruption in the communication between the temporal and frontal lobes of the brain may be the reason that Parkinson’s patients have hallucinations.

If you see a ghostly apparition out of the corner of your eye, then, it is best to remain skeptical and think about what other things it could be instead.

“I’d advise anyone wanting to approach the topics covered within parapsychology and anomalistic psychology to arm themselves with critical thinking skills. Such skills, they will find, can also be usefully applied in many other areas of life besides just ‘the weird stuff’!” Chris French told IFLScience in the 22nd issue of CURIOUS magazine.