What do we dream?

Every one of us slips into this mysterious state of consciousness every night, yet we are only now waking up to its mind-altering powers

“THE interpretation of dreams is the royal road to a knowledge of the unconscious activities of the mind.” So wrote Sigmund Freud in his 1900 classic The Interpretation of Dreams. He saw this idea as a “once in a lifetime” insight, and for much of the 20th century the world agreed. Across the globe, and upon countless psychoanalysts’ couches, people recounted their dreams in the belief that they contained coded messages about repressed desires. Dreams were no longer supernatural communications or divine interventions – they were windows into the hidden self.

Every one of us slips into this mysterious state of consciousness every night, yet we are only now waking up to its mind-altering powers

“THE interpretation of dreams is the royal road to a knowledge of the unconscious activities of the mind.” So wrote Sigmund Freud in his 1900 classic The Interpretation of Dreams. He saw this idea as a “once in a lifetime” insight, and for much of the 20th century the world agreed. Across the globe, and upon countless psychoanalysts’ couches, people recounted their dreams in the belief that they contained coded messages about repressed desires. Dreams were no longer supernatural communications or divine interventions – they were windows into the hidden self.

Today we interpret dreams quite differently, and use far more advanced techniques than simply writing down people’s recollections. In sleep laboratories, dream researchers hook up volunteers to EEGs and fMRI scanners and awaken them mid-dream to record what they were dreaming. Still tainted by association with psychoanalysis, it is not a field for the faint-hearted. “To say you’re going to study dreams is almost academic suicide,” says Matt Walker at the University of California, Berkeley. Nevertheless, what researchers are finding will make you see your dreams in a whole new light.

Modern neuroscience has pushed Freud’s ideas to the sidelines and has taught us something far more profound about dreaming. We now know that this peculiar form of consciousness is crucial to making us who we are. Dreams help us to consolidate our memories, make sense of our myriad experiences and keep our emotions in check.

Changing patterns of electrical activity tell us that the sleeping brain follows 90-minute cycles, each consisting of five stages – two of light sleep at the start, then two of deep sleep, followed by a stage of REM, or rapid eye movement sleep (see diagram). There is no characteristic pattern of brain activity corresponding to dreaming, but as far as we know all healthy people do it. And while dreaming is commonly associated with REM sleep, during which it occurs almost all of the time, researchers have known since the late 1960s that it can also occur in non-REM sleep – though these dreams are different. Non-REM dreams tend to be sparse and more thought-like, often without the complexity, length and vivid hallucinatory quality of REM dreams.

Despite their differences, both types of dreams seem to hold a mirror to our waking lives. Dreams often reflect recent learning experiences and this is particularly true at the start of a night’s sleep, when non-REM dreaming is very common. Someone who has just been playing a skiing arcade game may dream of skiing, for example (Sleep, vol 33, p 59). The link between waking experience and non-REM sleep has also been observed in brain scanning studies. Pierre Maquet at the University of Liège, Belgium, looked at the later stages of non-REM sleep and found that the brains of volunteers replayed the same patterns of neural activity that had earlier been elicited by waking experiences (Neuron, vol 444, p 535). Many REM-sleep dreams also reflect elements of experiences from the preceding day, but the connection is often more tenuous – so someone who has been playing a skiing game might dream of rushing through a forest or falling down a hill.

Sleep on it

But we do not simply replay events while we dream, we also process them, consolidating memories and integrating information for future use. Robert Stickgold of Harvard Medical School in Boston recently found that people who had non-REM dreams about a problem he had asked them to tackle subsequently performed better on it (Current Biology, vol 20, p 1). Likewise, REM sleep has been linked with improved abilities on video games and visual perception tasks, and in extracting meaning from a mass of information (Neurobiology of Learning and Memory, vol 92, p 237).

“It’s clear that the brain does an immense amount of memory processing while we sleep – and it certainly isn’t mere coincidence that while our brain is sorting out these memories and how they fit together, we’re dreaming,” says Stickgold. He suspects that the two types of dream states have different functions for memory, though what these functions are is a matter of debate. Non-REM dreaming might be more important for stabilising and strengthening memories, Stickgold suggests, while REM dreaming reorganises the way a memory is stored in the brain, allowing you to compare and integrate a new experience with older ones.

Jan Born and Susanne Diekelmann at the University of Lübeck in Germany, however, have looked at the same evidence and come to the opposite conclusion – that REM sleep supports the strengthening of a new memory, while non-REM sleep is for higher-level consolidation of memories (Nature Reviews Neuroscience, vol 11, p 114). “I think this means that we’re still lost when it comes to understanding the role of different sleep stages in memory,” says Stickgold.

Also unclear is how central is the role of dreams in memory formation. During dreaming is certainly not the only time our brains consolidate memories. For example, when we daydream certain areas of the brain, called the default network, become active. We now know this network is involved in memory processing (New Scientist, 8 November 2008, p 28) and many of the same brain regions are active during REM sleep. What’s more, daydreaming, like REM dreaming, can improve our ability to extract meaning from information and to have creative insights.

Does this mean we don’t actually need dream sleep to process memories? Not necessarily, says Walker, who points out that the way new memories are replayed in the brain is different in daydreaming and dreaming. Rat studies show that the reruns happen in reverse when the animals are awake and forwards when they are sleeping. No one is quite sure what this difference means for memory processing, but Walker believes it shows that daydreaming is not simply a diluted version of sleep dreaming. Maquet agrees. “Different brain states may all have somewhat different functions for memory. Memory consolidation is probably organised in a cascade of cellular events that have to occur serially,” he says – some while you are awake, and then some while you are asleep.

Even if dreaming is crucial for memory, Walker for one does not see this as its main function. “I think the evidence is mounting in favour of dream sleep acting as an emotional homeostasis: basically, rebalancing the emotional compass in a good way at the biological level,” he says. Everyone knows how a short nap can transform a cantankerous 2-year-old and Walker has shown something similar in adults. He found that a nap that includes REM dreaming mitigates a normal tendency in adults to become more sensitive to angry or fearful faces over the course of a day, and makes people more receptive to happy faces (Cerebral Cortex, vol 21, p 115).

Walker has also found that sleep, and REM sleep in particular, strengthens negative emotional memories (Cerebral Cortex, vol 19, p 1158). This might sound like a bad thing – but if you don’t remember bad experiences you cannot learn from them. In addition, both he and Stickgold think that reliving the upsetting experience in the absence of the hormonal rush that accompanied the actual event helps to strip the emotion from the memory, making it feel less raw as time goes on. So although dreams can be highly emotional, Walker believes they gradually erode the emotional edges of memories. In this way, REM dreams act as a kind of balm for the brain, he says. In people with post-traumatic stress disorder this emotion-stripping process seems to fail for some reason, so that traumatic memories are recalled in all their emotional detail – with crippling psychological results (New Scientist, 21 February 2009, p 34).

As with memory processing, REM and non-REM dreaming may play different psychological roles. Patrick McNamara of Boston University has found that people woken at different sleep stages give different reports of their dreams. REM dreams contain more emotion, more aggression and more unknown characters, he says, while non-REM dreams are more likely to involve friendly encounters (Psychological Science, vol 16, p 130). This has led him to speculate that non-REM dreams help us practise friendly encounters while REM dreams help us to rehearse threats (see “The interpretation of nightmares”).

So what do they mean?

All this suggests that we couldn’t function properly without dreaming, but it doesn’t answer the perennially intriguing question: what do dreams actually mean?

For some sleep researchers the answer is simple – and disappointing. Born argues that dreams themselves have no meaning, they are just an epiphenomenon, or side effect, of brain activity going on during sleep, and it is this underlying neuronal activity, rather than the actual dreams, that is important. Walker finds it hard to disagree. “I don’t want to believe it. But I don’t see large amounts of evidence to support the idea [that dreams themselves are significant],” he says.

Those researchers who refuse to accept the notion that the content of dreams is unimportant point to work by Rosalind Cartwright of Rush University, Chicago. In a long series of studies starting in the 1960s, she followed people who had gone through divorces, separations and bereavements. Those who dreamed most about these events later coped better, suggesting that their dreams had helped. “Cartwright’s work provides some of the most solid evidence that dreaming serves a function,” says Erin Wamsley at Harvard Medical School. There is no hard data showing that dreaming is not an epiphenomenon, she admits, but the same could be said about waking consciousness.

In fact, Wamsley’s own research hints that the form and function of a dream are connected. She worked with Stickgold on the study which found that non-REM dreams boost people’s performance on a problem. Their volunteers were given an hour’s training on a complex maze then either allowed a 90-minute nap or kept awake. The dreamers subsequently showed bigger improvements, but the biggest gains of all were in people who dreamed about the maze. It didn’t seem to matter that the content of these dreams was obtuse. One volunteer, for example, reported dreaming about the maze with people at checkpoints – though there were no people or checkpoints in the real task – and then about bat caves that he had visited a few years earlier. Stickgold didn’t expect this to improve the volunteer’s ability to navigate the maze, “and yet this person got phenomenally better”.

He points out that the dream content is consistent with the idea that during dreaming memories are filed with other past experiences for future reference. “Dreams have to be connected in a meaningful, functional way to improvements in memory – not just be an epiphenomenon,” he says. “I say this with fervent emotion, which is what I use when I don’t have hard data.”

Such evidence may one day be forthcoming, though. In the past there has been no objective way to record what someone is dreaming, but that could change. Yukiyasu Kamitani at the ATR Brain Information Communication Research Laboratory in Kyoto, Japan, and colleagues have used fMRI scans to recreate scenes that volunteers were picturing in their mind while awake (newscientist.com/article/dn16267). The team hasn’t yet done this with dreams, but it is theoretically possible, says Kamitani. It wouldn’t be like watching a movie, he adds, “but it may be possible to predict what kind of dream a person is experiencing using currently available technology”.

Some may think all this peering and prodding at our dream world is taking away its magic but the researchers don’t see it that way. While you are dreaming your brain literally reshapes itself by rewiring and strengthening connections between neurons. So although dreams do not reveal the secret you, they do play a key role in making you who you are. “The mystery and the wonder of dreams is untouched by the science,” says Stickgold. “It just helps us appreciate better how amazing they really are.”

The interpretation of nightmares

Antii Revonsuo enjoys his nightmares. “At least in hindsight,” he qualifies, “as though they were good horror movies where you don’t know it’s a movie until it’s over.” But then Revonsuo, at the University of Turku in Finland, thinks that nightmares are the main biological reason for why we dream – they allow us to simulate scary encounters, and so be better prepared for them in our waking life.

“The theory predicts correctly several features of our dream content,” says Revonsuo. For example, he and his colleagues have found that about two-thirds of the dreams of healthy adults involve at least one threat. About 40 per cent of these take the form of aggressive encounters – running away from an attacker or getting into a fight. Such encounters are higher among children, accounting for over half of threat dreams in Finnish kids and three-quarters among traumatised Palestinian children.

Revonsuo argues that children’s dreams are closer to our evolutionarily original form of dreaming because children haven’t yet had a chance to adjust to the modern environment. He has found that between 40 and 50 per cent of children’s dreams contain animal characters, often as enemies, which is similar to the instance among adult hunter-gatherers. The figure is just 5 per cent in western adults. “I don’t think any other dream theory has made such specific predictions and shown that they hold,” he says.

It is a neat idea, but Robert Stickgold at Harvard Medical School in Boston cannot believe that’s all there is to dreaming. “I think Revonsuo has made the same mistake as Freud – which is to limit dreaming’s functionality. I think dreaming is absolutely about threat rehearsal some of the time. But it’s absolutely about other things, too.”

Emma Young is a writer based in Sydney, Australia

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