Friday, June 19, 2:12 a.m.: Loading up the trunk of my car with clothes hangers when approached by two transients… try to engage them in good-natured conversation about the benefits of wooden clothes hangers over metal ones, but they make me uneasy, say they want to go out to get a drink but I’ve got to go. In a city somewhere… looks like a post-apocalyptic Saint Louis.

Saturday, June 20, 4:47 a.m.: Was just now trying to return my dead grandmother’s cane to her. Took elevator to her apartment… meant to go to the 8th floor, but elevator lurched up to the 18th floor, swung around violently then shot back down. Could hear voices in the corridors outside elevator shaft…. a mother yelling at her child. Grandma then became my other grandma, also deceased, yet in a nursing home; doctors say she’s doing fine.

Sunday, June 21, 5:02 a.m.: On a floating barge in the sea trying to get to some other country, just made it, the dogs are running all over the place but seem more like rodents.

Monday, June 22, 3.31 a.m.: Just learned that one of my colleagues died suddenly, everyone’s in shock (they say it was “an accidental overdose of oxygen from a breathing tank; he fell asleep”). Can’t believe it, was just talking to him today about death. Also something about an airplane delay… need to get home but can’t find my test results to submit, searching all over, trash cans, pulling out drawers… people preoccupied.

These are dreams, of course. Mine from the past few days, to be precise—and they are totally absurd. Why on earth do our minds conjure up such ridiculous imagery, such inane thoughts, such spectacularly vivid and surreal landscapes, intense emotions—such narrative trash?

Over the years, many psychologists have conjured up their own evolutionary explanations. And it’s a fair assessment of the literature to say that we still don’t know why we dream. After all, although it’s relatively easy to see why sleep itself would have conferred evolutionary advantages (avoiding nocturnal predators, recharging our neural batteries and so on), it’s not entirely clear why we don’t simply sleep without dreaming.

Harvard University psychologist Deirdre Barrett might have some answers for us. In a recent review of evolutionary theories concerning the possible adaptive function of dreaming, Barrett shrugs off the better-known psychoanalytic theories of dreams (for example, Freud’s “wish fulfillment” and Jungian archetypes) as being irreconcilable with a Darwinian framework and instead highlights the major contemporary, biologically informed theories. Remember, the key question for us to consider is why dreaming occurs at all, since it’s not immediately apparent why natural selection wouldn’t have simply engineered a dreamless, non-REM sleep.

So, without further ado, here’s a snapshot of the current evolutionary contenders in the field of dream studies:

Brain Conditioning

In the 1960s, sleep researchers began to notice that, in contrast to other stages of sleep, the brain was especially active during REM sleep. Several researchers, including the psychophysiologist Fred Snyder, argued that the adaptive purpose of dreaming may therefore be primarily to stimulate the brain or to keep it “in shape” during prolonged periods of inactivity. Later research offered support for this general idea. For example, specific categories of neurotransmitters were shown to be highly active during this period, while others seemingly “rested.” And specific anatomical regions of the brain, too, were especially busy during REM sleep (especially the amygdala), whereas others showed a pattern of reduced activity compared to waking (the prefrontal lobes, parietal cortex and posterior cingulate). Finally, thermoregulation is turned off during REM sleep.

All of this, Barrett notes, led many researchers to conclude that dreaming was necessary for one or more of these functions: to replenish neurotransmitters, rest a particular brain area, or restore the thermoregulatory system. In general, these brain conditioning theories don’t place much, if any, emphasis on dream content. As psychologist Steven Pinker writes, “For all we know, dreaming might be a kind of screen saver in which it doesn’t really matter what the content is as long as certain parts of the brain are active.”

External Vigilance

University of California at Santa Barbara anthropologist Don Symons wasn’t entirely satisfied by brain conditioning theories of dreaming, in large part because these theories didn’t really crack the question of why dreams have such a specific sensory profile of being so vividly visual and kinesthetic while comparatively impoverished in sound, smell and other sensory domains. Symons points out that sleepers are particularly vulnerable to real-world threats and hazards in their external environment, and so they must unconsciously monitor their environment with specific senses. For example, if our ancestors were busy having olfactory or auditory hallucinations in their dreams that were equally as rich as their visual hallucinations occurring beneath their fluttering eyelids, well then they might not have noticed the ominous smell of smoke creeping up their nostrils or the threatening strangers or predators pattering around outside. Being a “light sleeper” in relation to these other sensory domains had adaptive benefits, and since we’re in the dark anyway and our eyes are closed, there’s less of a risk in hallucinating in our secret visual worlds while our brains are being recharged through the processes described in the previous section.

Threat Simulation Theory

Originally proposed by Finnish neuroscientist Antti Revonsuo, this clever evolutionary theory holds that dreaming serves a biologically adaptive function because it allowed our ancestors to simulate problem-solving strategies for genuine, waking life threats. Antonio Zadra, Sophie Desjardins, and Eric Marcotte of the University of Montreal neatly summarize the central argument of the theory this way: “By giving rise to a full-scale hallucinatory world of subjective experience during sleep, the dream production mechanism provides an ideal and safe environment for such sustained practice by selecting threatening waking events and simulating them repeatedly in various combinations.” What we should see in contemporary dreams, argues Revonsuo, are “threat scripts” depicting primitive themes of danger that would likely have been relevant in the ancestral environment, such as being chased, falling and so on.

Costly Signaling Theory

Boston University neuroscientist Patrick McNamara has an interesting evolutionary theory of dreaming. McNamara’s theory draws from the well-known “handicapping principle” in evolutionary biology, where some organisms have been observed to display behavioral traits or physical characteristics that seem ostensibly to disadvantage them but in fact simply reflect their genetic value. The classic example of this is “stotting” behavior in healthy young gazelles, where these animals jump up and down in front of a predatory leopard rather than—what would seem to be a smarter move—immediately running away. Stotting is a “costly signal,” but it works, because the leopards take this stotting display as evidence that this particular gazelle is so healthy and fit that it can afford to handicap itself and is therefore unlikely to be an easy target. Usually, the leopard moves onto the sick, old, or young non-stotters.

McNamara argues that dreaming can best be understood also as a type of costly signal. He points out that REM sleep is associated with increased mortality, particularly with respect to the irregular activity of the cardiovascular system. He also notes that human males “waste” a lot of otherwise good penile erections during REM sleep. Also, many of the awkward, embarrassing, anxiety-producing experiences from our more negative dreams tend to filter into our waking life, leaving a sort of lingering emotional residue that puts us at an adaptive disadvantage by compromising our everyday social interactions. The bottom line, according to McNamara, is that if we can “afford” to have REM sleep and still be players in the reproductive game, then we’re essentially communicating to others (presumably, others who are watching us sleeping or infer that we’ve done so) that we’re in possession of high-quality genes.

Dreaming as Problem-Solving

Barrett’s preferred evolutionary explanation for dreaming, and the one she’s best known for, is that dreamscapes provided our ancestors (and therefore us) with a sort of creative canvas for solving real-world problems. In support of this, Barrett describes the work of Stanford University psychologist William Dement, who in the early 1970s instructed hundreds of undergraduate students to work on a set of challenging brainteasers before bedtime, so that they’d fall asleep with the problems still on their mind. For example, “The letters O, T, T, F, F … form the beginnings of an infinite sequence. Find a simple rule for determining any or all successive letters.” [The correct sequence is the first letter of each number, so the next one would be “S” for “six.”] One participant who went to bed frustrated by this brainteaser dreamed:

I was walking down the hall of an art gallery. I began to count the paintings—one, two, three, four, five. But as I came to the sixth and seventh, the paintings had been ripped from their frames! I stared at the empty frames with a peculiar feeling that some mystery was about to be solved. Suddenly I realized that the sixth and seventh spaces were the solution to the problem.

Barrett also cites many examples of notable figures from all walks of life (including scientists) who similarly arrived at their groundbreaking discoveries and insights through dreams. These include the German chemist August Kekulé’s famous dream of a snake grabbing its own tail leading to his discovery of the benzene molecule structure, and Demitri Mendeleev’s literal dream of the periodic table for classifying chemical elements. Such anecdotes, as well as an impressive range of experimental findings, suggest to Barrett that a simple brain conditioning explanation for the existence of dreaming is shortsighted. In some conditions, she notes, "sleeping on it" may be better than waking thought.

Personally, I think each of these major evolutionary theories of dreaming has some currency. However, none of them directly addresses the puzzling phenomenon of recurring dreams—you know, the ones you have over and over again through the years, playing like a broken record. (Mine is the stereotypical forgetting of my high school locker combination or class schedule; odd since neither seemed to faze me much at the time.) In a 2006 study published in the journal Consciousness and Cognition, Zadra, Desjardins, and Marcotte performed a content analysis on a set of 212 recurrent dreams reported by participants ranging from 18-81 years of age. Among their findings, escape and pursuit themes were the most frequent type of threat found in their sample (25.9 percent), followed by accidents and misfortunes (19.7 percent), aggression and violence (19.0 percent), physical difficulties (17.0 percent), emotional difficulties (7.5 percent), and disasters (3.4 percent). Furthermore, in nearly all cases the dreamer him- or herself (rather than a stranger or loved one) was the specific target of the threat and usually the dreamers actively participated in some way to resolve, escape, or combat the threat.

The authors interpret these recurring dream findings in light of Threat Simulation Theory, but they also question the theory’s ability to account sufficiently for such a wide range of bizarre, surreal and non-realistic threats. (Especially those that wouldn’t have posed adaptive problems for our ancestors.) However, Revonsuo and his colleague Katja Valli defend Threat Simulation Theory on this score, saying that, “Fantasy-based threats can activate the threat perception and avoidance mechanisms in a relevant manner, just as effectively as reality-based simulations. For dreaming to function as an efficient threat simulation it makes little difference whether it is a realistic wolf or a werewolf chasing you in the dream.”

Finally, speaking of bizarre and surreal, there’s of course the curious case of erotic dreams. But perhaps that’s a subject best saved for another post.

In this column presented by Scientific American Mind magazine, research psychologist Jesse Bering of Queen's University Belfast ponders some of the more obscure aspects of everyday human behavior. Ever wonder why yawning is contagious, why we point with our index fingers instead of our thumbs or whether being breastfed as an infant influences your sexual preferences as an adult? Get a closer look at the latest data as “Bering in Mind” tackles these and other quirky questions about human nature. Sign up for the RSS feed or friend Dr. Bering on Facebook and never miss an installment again.