A Season in Hell
As a grad student in psychology in the early 1990s, Revonsuo often had bad dreams. What struck him the most was how lifelike they were. "I would say to myself, in my dream, 'Oh shit! I've dreamt of this before, but now this is really happening!' " he recalls.
"Credible world analogs" are what cognitive psychologist David Foulkes calls dreams. Although we tend to dwell on the bizarreness of dreams, most dreams are quite mundane, Foulkes notes. You move around, talk, run, interact with others, experience emotions, and feel the passage of time, just as in everyday life.
When Revonsuo began studying dreams, he asked his students to start keeping logs of their own nocturnal escapades. He noticed something striking. The dreams were filled with dangerous events, negative emotions, monsters, chases, escapes, fights, and near-death experiences. The dream world was a hellscape of danger, teeming with threatening events far more sinister than in waking life.
These weren't the misfirings of diseased brains. Threat dreams were the norm, accounting for a staggering two-thirds of all dreams. Revonsuo discovered that we grossly underestimate the number of nightmares we have. As it turns out, we have 300 to 1,000 threat dreams per year—one to four per night. Just under half are aggressive encounters: physical aggression such as fistfights, and nonphysical aggression such as verbal arguments. The rest are about car crashes, falling and drowning, missing a meeting or a test, being lost or trapped, and being naked in public. The whole dream world seemed to have a negative bias: more negative emotions than positive ones, more misfortune than good fortune, more nightmares than fantasy.
A Theory Is Born
In the ancestral environment, Revonsuo reasoned, our dreams served to protect us, teaching us how to respond when a wild animal was chasing us or when we got lost in the forest. That was why the dream world was so filled with peril: to simulate the potential threats and prepare us to react quickly. But how could dreams help us select the optimal response, given that dream recall is so fragile? After all, we remember only a few of our dreams, and even those fade fast in the tumult of the day.
Revonsuo believes that by providing rehearsal, dreaming helps us recognize dangers more quickly and respond more efficiently. We don't need to be aware of this rehearsal, just as you don't have to recall exactly where you practiced your tennis serve in order to reap the rewards.
The idea that dreams are a dojo for perfecting waking activities fits well with what is already known about practice. Mental rehearsal through visualization improves skills, enhances learning, and changes the brain, polishing performance in almost any domain, from sports to piano playing.
The single most pervasive theme in dreaming is that of being chased or attacked. Just as athletes in training repeat parts of their performance, we may, in our nightmares, be attacked and chased over and over again, not to solve a particular problem but to actually practice efficient escape behavior.
Saber-toothed tigers no longer stalk our villages, but Stone Age themes still rule our dreams. "Nowadays, the evolutionary footprint is clearest in the dreams of children, who often dream about being chased by monsters, much the same way we were once chased by predators," says Revonsuo. As life has evolved, so have the threats we rehearse. "You insert a modern danger into that ancestral key and get a bizarre combination," says Revonsuo. "We dream of being chased, shot, or robbed, getting into traffic accidents, a burglar in our house, or perhaps smaller mishaps such as losing our wallets—and that prepares us for our waking life."
The dreaming brain, explains Revonsuo, scans emotional memories. When it detects a memory trace with a strong negative emotion, it constructs a nightmare around that theme. The more traumatic the event, the more intense the nightmare. The brain's system for detecting threats is sensitive and flexible: Anything the brain tags with a strong negative charge gets thrown into the threat bin and dredged up at night.
Sometimes this system works well: Dreaming about a boy running in front of our car better prepares us should that danger crop up in real life. But sometimes the modern world throws the threat-detection mechanism out of whack: Watching horror movies can trigger nightmares about vampires, ghosts, aliens, or zombies. Such "nonsense nightmares" don't rehearse any useful threats; they're like an allergic reaction, says Revonsuo. Just as our immune system can mistake pollen for a pathogen and mount a defensive campaign, the threat-detection system misperceives horror movies and deploys its defenses by generating a nightmare.
Heroes of Our Own Dreams
In the jungles of the Amazon lives a tribe called the Mehinaku. The Mehinaku lead the traditional life of hunter-gatherers. They spend their days fishing and gathering roots. Since they believe that dreams predict the future, they are scrupulous about remembering them and sharing them with others. That makes them perfect for an ethnographic study of dreams. In 1981, anthropologist Thomas Gregor surveyed their dreams and analyzed the content.
As it turns out, the Mehinaku dream profusely about the dangers in their everyday lives: being attacked by wild pigs; chased by jaguars; bitten by snakes; stung by wasps, ants, or bees—all potentially lethal. "Their dreams simulate over and over again what to do and how to do it quickly when they spot these animals in the wild," reports Revonsuo. Across a tribesman's lifespan, a single failure to react efficiently could be fatal. If threat simulation even marginally increases the likelihood that such fatal failures won't occur, it would prove adaptive.
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