Dreaming person
© Gaetan Charbonneau/Getty Images
Many people report that they have had premonitions of disasters in their dreams.
When disaster strikes, people often claim that they foresaw the tragedy. But are such premonitions really possible? In an extract from his new book Paranormality, psychologist Richard Wiseman explains how our sleeping minds can trick us.

Aberfan is a small village in south Wales. In the 1960s, many of those living there worked at a nearby colliery that had been built to exploit the large amount of high-quality coal in the area. Although some of the waste from the mining operation had been stored underground, much of it had been piled on the steep hillsides surrounding the village. Throughout October 1966 heavy rain lashed down on the area and seeped into the porous sandstone of the hills. Unfortunately, no one realised that the water was then flowing into several hidden springs and slowly transforming the pit waste into soft slurry.

Just after nine o'clock on the morning of 21 October, the side of the hill subsided and half a million tonnes of debris started to move rapidly towards the village. Although some of the material came to a halt on the lower parts of the hill, much of it slid into Aberfan and smashed into the village school. A handful of children were pulled out alive during the first hour or so of the rescue effort, but no other survivors emerged. One hundred and thirty-nine schoolchildren and five teachers lost their lives in the tragedy.

Psychiatrist John Barker visited the village the day after the landslide. Barker had a longstanding interest in the paranormal and wondered whether the extreme nature of events in Aberfan might have caused large numbers of people to experience a premonition about the tragedy. To find out, Barker arranged for a newspaper to ask any readers who thought they had foreseen the Aberfan disaster to get in touch. He received 60 letters from across England and Wales, with over half of the respondents claiming that their apparent premonition had come to them during a dream.

One of the most striking experiences was submitted by the parents of a 10-year-old child who perished in the tragedy. The day before the landslide their daughter described dreaming about trying to go to school, but said that there was "no school there" because "something black had come down all over it". In another example, Mrs MH, a 54-year-old woman from Barnstaple, said that the night before the tragedy she had dreamed that a group of children were trapped in a rectangular room. In her dream, the end of the room was blocked by several wooden bars and the children were trying to climb over the bars. Another respondent, Mrs GE from Sidcup, said that a week before the landslide she had dreamed about a group of screaming children being covered by an avalanche of coal, and two months before the tragedy Mrs SB from London had dreamed about a school on a hillside, an avalanche and children losing their lives. And so the list went on.

Believing that you have seen the future in a dream is surprisingly common, with recent surveys suggesting that around a third of the population experience this phenomenon at some point in their lives. Abraham Lincoln reportedly dreamed about an assassination two weeks before being shot dead, Mark Twain described a dream in which he saw his brother's corpse lying in a coffin just a few weeks before he was killed in an explosion, and Charles Dickens dreamed of a woman dressed in red called Miss Napier shortly before being visited by a girl wearing a red shawl and introducing herself as Miss Napier.

What could explain these remarkable events? Are people really getting a glimpse of things to come? Is it possible to see tomorrow today? It is only in the past century or so that researchers have managed to solve the puzzle.

In the 1950s the pioneering US psychologist Eugene Aserinsky helped pave the way for a new science of dreaming. He showed that waking up a person after they have spent some time in the REM state is very likely to result in them reporting a dream. The decades of work that followed have yielded many important insights. Almost everyone dreams in colour. Although some dreams are bizarre, many involve everyday chores such as doing the washing-up, filling in tax forms, or vacuuming. If you creep up on someone who is dreaming and quietly play some music, shine a light on their face or spray them with water, they are very likely to incorporate the stimuli into their dreams. However, perhaps the most important revelation was that you have many more dreams than you might think.

Aberfan rescue effort
© Bentley Archive/Popperfoto/Getty
The rescue effort after the Aberfan mining disaster in October 1966.
Sleep scientists quickly discovered that you have an average of about four dreams each night. They take place every 90 minutes or so, and each one lasts around 20 minutes. You then forget the vast majority of these episodes when you wake up, leaving you with the impression that you dream far less than is actually the case. The only exception to this rule occurs when you happen to wake up during a dream. When this happens you will usually remember the gist of the dream and perhaps some specific fragments but, unless it is especially striking, you will soon forget all about it. There is, however, a set of circumstances that can greatly increase your likelihood of remembering these dreams. In a process similar to word association, an event that happens to you when you are awake can trigger the memory.

Let's imagine three nights of disturbed dreaming. On day one you go to bed after a hard day at work. Throughout the night you drift through the various stages of sleep and experience several dreams. At 10 past seven your brain once again bursts into action and presents you with another entirely fictitious episode. For the next 20 minutes you find yourself visiting an ice cream factory, falling into a huge vat of raspberry-ripple, and attempting to eat your way out. Just when you can take no more, your alarm clock sounds and you wake up with fragments of the factory and raspberry-ripple ice cream drifting through your mind.

On day two the same series of events unfolds. You go to bed, drift to sleep and have several dreams. At 2am you are right in the middle of a rather sinister dream in which you are driving along a dark country lane. Eric Chuggers, your all-time favourite rock star, is sitting in the passenger seat, and the two of you are chatting easily. Suddenly a giant purple frog jumps out in front of the car, you swerve to avoid the frog but go off the road and hit a tree. Back in the real world, your cat jumps on to the bed and you wake up from the dream with a vague memory of Eric Chuggers, a giant purple frog, a tree and impending death.

On the third night you again fall asleep. At 4am you experience a rather traumatic dream. It is a surreal affair, with you being forced to audition for the part of an Oompa Loompa in a new film version of Charlie and the Chocolate Factory. Although successful, you subsequently discover that the orange makeup and green hair dye used in the audition is permanent. You suddenly wake up feeling very stressed, and spend the next 20 minutes trying to figure out the symbolic meaning of the dream.

In the morning you wake up, turn on the radio and are shocked to discover that Eric Chuggers was killed in a car accident during the night. According to the news report, Chuggers was driving through the city, swerved to avoid another car that had drifted on to the wrong side of the road, and collided with a lamp-post. Bingo. This news report acts as a trigger, and the dream about the car accident jumps into your mind. You forget about consuming copious amounts of raspberry-ripple ice cream, and the stressful Oompa Loompa audition. Instead, you remember the one dream that appears to match events in the real world and so become convinced that you may well possess the power of prophecy.

And it doesn't stop there. Because dreams tend to be somewhat surreal they have the potential to be twisted to match the events that actually transpired. In reality, Chuggers was not driving along a country lane, did not hit a tree and the accident didn't involve a giant purple frog. However, a country lane is similar to a city road, and a lamp-post looks a bit like a tree. And what about the giant purple frog? Well, maybe that symbolised something unexpected, such as the car that drifted on to the wrong side of the road. Or maybe it turns out that Chuggers was on hallucinogenic drugs and so might have thought that the oncoming car was indeed a giant purple frog. Or maybe Chuggers's next album was going to have a frog on the cover. Or maybe Chuggers was wearing a purple shirt at the time of the collision. You get the point. Provided that you are creative and want to believe that you have a psychic link with the recently deceased Mr Chuggers, the possibilities for matches are limited only by your imagination.

You have lots of dreams and encounter lots of events. Most of the time the dreams are unrelated to the events, and so you forget about them. However, once in a while one of the dreams will correspond to one of the events. Once this happens, it is suddenly easy to remember the dream and convince yourself that it has magically predicted the future. In reality, it is just the laws of probability at work.

This theory also helps explain a rather curious feature of precognitive dreaming. Most premonitions involve a great deal of doom and gloom, with people regularly foreseeing the assassination of world leaders, attending the funeral of close friends, seeing planes fall out of the sky, and watching as countries go to war. People rarely report getting a glimpse of the future and seeing someone deliriously happy on their wedding day or being given a promotion at work. Sleep scientists have discovered that around 80% of dreams are far from sweet, and instead focus on negative events. Because of this, bad news is far more likely than good news to trigger the memory of a dream, explaining why so many precognitive dreams involve foreseeing death and disaster.

At the start of this article I described how psychiatrist John Barker found 60 people who appeared to have predicted the Aberfan mining disaster. In 36 of Barker's cases the respondents provided no evidence that they had recorded their dream prior to the disaster. These respondents may have had many other dreams before hearing about Aberfan, and then only remembered and reported the one dream that matched the tragedy. Not only that, but the lack of any record made at the time of the dream means that they could have inadvertently twisted the dream to better fit the unfortunate events that transpired. Blackness may have become coal, rooms may have become classrooms, and rolling hillsides may have become a Welsh valley.

Of course, those who believe in paranormal matters might argue that they are convinced by instances when people tell their friends and family about a dream, or describe it in a diary, and then discover that it matches future events. In the late 1960s researchers found that the content of our dreams is not only affected by events in our surroundings, but also often reflects whatever is worrying our minds. This may explain one of the most striking examples of alleged precognition about the Aberfan disaster. We have heard how one of the young girls who would later perish in the tragedy told her parents that she had dreamed about "something black" coming down over her school and the school no longer being there. For several years before the disaster the local authorities had expressed considerable concern about the wisdom of placing large amounts of mining debris on the hillside, but their worries had been ignored by those running the mine.

Three years before the disaster, the borough engineer in the area wrote to the authorities noting: "I regard [the situation] as extremely serious as the slurry is so fluid and the gradient so steep that it could not possibly stay in position in the winter time or during periods of heavy rain," and later added, "this apprehension is also in the minds of . . . the residents in this area as they have previously experienced, during periods of heavy rain, the movement of the slurry to the danger and detriment of people and property." There is no way of knowing for sure, but it is possible that the young girl's dream may have been reflecting these anxieties.

But what about the other 23 cases in which people produced evidence that they had described their dream before the tragedy occurred, and where the dream did not seem to reflect their anxieties and concerns. To investigate, we need to move away from the science of sleep and into the heady world of statistics. Let's take a closer look at the numbers associated with these seemingly supernatural experiences.

First, let's select a random person from Britain and call him Brian. Next, let's make a few assumptions about Brian. Let's assume that Brian dreams each night of his life from age 15 to 75. There are 365 days in each year, so those 60 years of dreaming will ensure that Brian experiences 21,900 nights of dreams. Let's also assume that an event like the Aberfan disaster will only happen once in each generation, and randomly assign it to any one day. Now, let's assume that Brian will only remember dreaming about the type of terrible events associated with such a tragedy once in his entire life. The chances of Brian having his "disaster" dream the night before the actual tragedy is about a massive 22,000 to one. However, here comes the sneaky bit. In the 1960s there were around 45 million people in Britain, and we would expect one person in every 22,000, or roughly 2,000 people, to have this amazing experience in each generation. The principle is known as the Law of Large Numbers, and states that unusual events are likely to happen when there are lots of opportunities for that event.

Our example only concerned people dreaming about the Aberfan tragedy. In reality, national and international bad fortune happens on an almost daily basis. Aeroplane crashes, tsunamis, assassinations, serial killers, earthquakes, kidnappings, acts of terrorism, and so on. Given that people dream about doom and gloom more often than not, the numbers quickly stack up and acts of apparent prophecy are inevitable.

Extracted from Paranormality: Why We See What Isn't There by Professor Richard Wiseman, published by Macmillan on 4 March 2011 at £12.99.