Creaking floorboards, things ‘bumping’ in the night and the creeping feeling that you are not alone.
An astonishing third of people in Britain and almost half of Americans say they believe in ghosts, spirits and other forms of paranormal activity.
Meanwhile, one in five say they have actually seen an apparition and one in 10 have communicated directly with the dead, surveys show.
But could there be a more down-to-earth medical explanation for these eerie phenomena?
Yes, say experts, who suggest a bizarrely common condition known as sleep paralysis as a major reason for observed ghost phenomena.
According to the NHSWith sleep paralysis, you are unable to move or speak while you wake up or fall asleep.
It occurs at a specific stage of the sleep cycle known as REM sleep, when the brain is very active but the body is still ‘asleep’.
The most vivid dreams are thought to occur during this stage and sleep paralysis is believed to occur when people wake up during this stage.
One in five say they have actually seen an apparition and one in ten have communicated directly with the dead, surveys show.
Some patients with the condition also report a feeling of pressure in the chest and may experience hallucinations, including frightening figures.
Descriptions of the condition can be found in folklore dating back hundreds of years, when it was given names such as ‘old witch’ – due to the idea that a witchy spirit sat on the patient’s chest.
Modern science cannot explain why this occurs, but it is known to be more common in people suffering from insomnia, post-traumatic stress disorder and anxiety.
Another cause of ghost sightings could be much more concerning: carbon monoxide.
The colorless, odorless gas can be emitted from poorly installed, defective or improperly used appliances such as water heaters, gas stoves and grills.
It is poisonous and can cause a range of symptoms including headache, confusion, dizziness and nausea.
Carbon monoxide can also cause hallucinations, which medical experts have linked to reports of ghostly events as far back as the 1920s.
Seeing ghosts may also be related to a brain abnormality, studies suggest.
Experts suggest that a bizarre condition known as sleep paralysis is a major reason for perceived hauntings
In 2017, a leading medical journal published the story of a 26-year-old patient with epilepsy who underwent an experimental treatment that involved implanting electrodes in his brain.
When clinicians at the New York State Department of Health’s National Center for Adaptive Neurotechnologies simulated a part of the brain known as the fusilform face area, or FFA, he experienced terrifying hallucinations.
“Your face changed completely,” he reportedly said as he stared at one of the investigators. ‘I don’t know what’s going on. Your eyes… change.”
He described the people around him as “anime characters,” referring to the Japanese cartoon genre.
Stimulating the FFA also caused rainbows to appear around objects, the researchers added.
A separate experiment in another epilepsy sufferer – a 22-year-old woman – found that stimulating the left hemisphere of the brain caused her to feel as if ‘a person’ or ‘shadow’ who was not moving or speaking was directly behind her .
These findings could provide some insight into why up to half of people diagnosed with the neurological condition Parkinson’s – which causes progressive brain damage – also claim to see ghostly figures.
Meanwhile, recent research has revealed the reason for half of people claim to ‘hear’ the voice of a deceased loved one during periods of mourning.
Recent research has revealed the reason why up to half of people claim to ‘hear’ the voice of a deceased loved one or feel their presence during periods of grief – just as Demi Moore’s character did in the 1990s classic Ghost , with Patrick Swayze.
This is thought to be due to the way the brain creates what psychology calls schemas: a mental framework that helps people organize and interpret information based on past experiences.
When we enter a restaurant, we know from experience that we will be greeted, then seated, then offered drinks and a menu, and so on.
Likewise, couples become so accustomed to each other’s presence at home – so much so that even when someone is away, it sometimes feels like they are still there.
Relatives also often describe that they can hear their husband or wife talking, according to grief counselors.
So if you see a ghost, don’t panic, think about what the other cause could be.
On the other hand, there may not be any credible scientific evidence for life after death… that doesn’t rule it out completely.