The REAL reasons why some people see ghosts, according to experts
Many people claim to have witnessed ghosts and ghouls - we explore some of the possible causes for all things supernatural
COLD spots, creaking sounds and spooky figures - whether or not the paranormal realm exists is an issue that has been debated for centuries.
But in a world filled with science and reason, these "hauntings" can often be boiled down to simple explanations - and it has nothing to do with the supernatural.
'Waking dream' and insomnia
Ever get that feeling when you're awake but you're having a terrifying dream and your body is frozen? This is called "waking dream" - a hypnagogic state when your mind is alert but your body is still asleep - and can be associated with sleep paralysis.
Joe Nickell, a senior research fellow for the Committee for Skeptical Inquiry - which promotes scientific inquiry and critical investigation of paranormal claims - believes ghosts are "all in the mind".
People are more prone to see a ghost when they are in an altered state of consciousness
Joe Nickell , Senior Research Fellow for Committee for Skeptical Inquiry
He told The Sun Online: "In the 'waking dream' state, people wake up and see a number of things - aliens, dead people, ghosts and are associated with feelings of dread.
“They’ll often see the entity coming into their room standing by their bed or trying to choke them and they’re not able to move or talk or scream or do anything.
"It's a trick of the mind when the mental image is being superimposed on the actual visual scene. So the imagined events seem very real."
The US-paranormal investigator with 50 years of experience believes a ghost can also be an illusion produced by the brain, particularly when a person suffers from insomnia.
He added: "The mind is not at its sharpest when we're tired or lost in thought.
"People are more prone to see a ghost when they are in an altered state of consciousness. Wondering through a spooky place while you're tired is a good recipe for a ghost."
A fear of ghosts
Founder of the American Paranormal Research Association (APRA) Brandon Alvis said phasmophobia - the fear of ghosts and the unknown - causes people to think they are encountering sinister entities.
Speaking to The Sun Online, the paranormal investigator said: "A majority of the time, people become so consumed by the thought of a ghost inhabiting their home that it affects the quality of life.
"It can cause panic attacks and keep people apart from loved ones.
"Symptoms typically include shortness of breath, rapid breathing, irregular heartbeat, sweating, nausea and the overall feeling of dread.
"Although everyone experiences fear of ghosts in their own way and may have different symptoms."
Dr Nickell, dubbed the "real-life Scully" from the X-Files, added: "Ghosts have to do with people's beliefs - people who believe in them are prone to have something happen.
"So when people go to a haunted place expecting to see ghost, they often see something moving in the corner of their eye - an illusion."
Deprivation of oxygen
Cerebral anoxia - a medical term for a lack of oxygen flowing to the brain - is said to trigger sensory distortions and hallucinations.
It is believed to be the physical means by which phenomena such as near-death experiences and out-of-body episodes might be explained.
Alvis, who authored Haunted Discoveries, said there are many cases of near-death experiences that can be explained by a lack of oxygen to the brain.
He added: "We also believe that people frequenting abandoned buildings with large amounts of mould and other hazardous elements can trigger cerebral anoxia, be that in a small or large scale.
"This would lead people to believe that they are experiencing something supernatural."
Carbon monoxide poisoning
The cause of all sorts of hallucinations - carbon monoxide poisoning has been linked to haunted houses since the 1920s.
Evidence from a research group led by Olaf Blanke shows the brain can trick people into feeling the "presence" of a ghostly apparition.
Carbon monoxide is odourless and colourless and is produced when you have a gas leak in your home.
The silent killer can be breathed in without you even knowing that it's there.
Symptoms include dizziness, nausea, tiredness and confusion, auditory hallucinations as well as shortness of breath.
More than 500 Americans die of carbon monoxide poisoning every year, but victims who survive can suffer from symptoms for years - even after their exposure ends.
The long term effects of breathing in the toxic substance can affect memory, brain function, behaviour and cognition.
In an interview with radio show This American Life, toxicologist Albert Donnay said: "Carbon monoxide poisoning can cause all manner of hallucinations- audio, visual, feeling strange things on their skin when there was nothing there.
"People often report that they hear noises in their ears, bells ringing, rushing sounds."
Electromagnetic waves
Apparitions, cold spots and ghostly touches can be caused by man-made magnetic fields, experts have suggested.
Electrical devices such as appliances, power lines, or even batteries all have an electromagnetic fields.
One in five American adults say they’ve seen or been in the presence of a ghost, according to Pew Research Center.
And about 29 per cent say they have felt in touch with someone who has already died.
APRA investigator Alvis said: "High electromagnetic fields and bad wiring can cause temporal lobe activity, making people believe that they are in the presence of ghosts.
"It is often associated with strange sensations, time distortions and hallucinations."