By Leslie Alan Horvitz
First of three parts
No one knows why we dream. It stands to reason that dreams have some purpose because nearly everyone dreams, and we dream 3 to 6 times every night. People typically have several dreams each night that grow longer as sleep draws to a close. Over a lifetime, a person may dream for five or six full years. The types of dreams you have change depending on your age and life circumstances.
Psychology Today offers a simple definition: “Dreams are the stories the brain tells during the REM (rapid eye movement) stage of sleep.” But why does the brain tell these stories to itself?
Dreams can be a portal into your mental state, says Dr. Rahul Jandial, neurosurgeon, neuroscientist, and author of This is Why You Dream: What Your Sleeping Brain Reveals About Your Waking Life. Some dreams, he believes, are worth paying attention to; others not so much. It’s worth considering the possible causes of an intense dream. “Dreams with a strong emotion and a powerful central image, those are ones not to ignore,” he says. Strong, negative emotional states like anxiety and stress are known to trigger bad dreams. This may be why up to 80 percent of those with PTSD experience frequent nightmares.
If they recur, anxiety dreams are among those we need to heed. When you’re experiencing more stress or anxiety, you tend to dream more, too. Nightmares or stressful dreams—for example, about being chased or being in a frightening situation—are also common when you’re anxious, says behavioral sleep medicine expert Michelle Drerup at the Cleveland Clinic. “That’s one of the theories of why we dream. Our dreams might help us process and manage our emotions.” Culture or societal norms may also be a factor, she notes. “There seems to be some cultural influence on dreams. For example, the same type of dream might be more common in Germany.”
“The dreaming brain is serving a function, and if it gives you a nugget of an emotional and visual dream, reflect on that,” Jandial notes. “That’s a portal to yourself that no therapist can even get to.”
Paying attention to powerful dreams is easier said than done since we forget, on average, 95 percent of our dreams when we wake up.
So, if we forget most of our dreams, why do we have them? Theories abound. Freud believed dreams express repressed content, ideas, or themes. Carl Jung, who famously broke with Freud over the role of the unconscious, maintained that “… the dream is a spontaneous self-portrayal, in symbolic form, of the actual situation in the unconscious.”
According to a more recent theory, dreaming can help you learn and develop long-term memories. In other words, dreams consolidate memory tasks and learning that have occurred during the day. In that respect, they perform a clerical function. Other theories see dreams as a unique state of consciousness that incorporates the present experience, processing of the past, and preparation for the future.
More pertinently, dreams may prepare us for possible future threats, which is related to the theory of dreams as a cognitive simulation of real-life experiences, a subsystem of a waking default network (involved in daydreaming). In other words, we can rehearse our feelings in our dreams. “… [D]reaming may represent important cognitive functioning,” says Drerup, “Brain activity that occurs when we’re dreaming is similar to the memory processing brain activity we experience when we’re awake.” Dreams may also provide a “safe space” where problems that are too overwhelming, contradictory, or disturbing to deal with by the conscious mind adequately can be integrated and resolved.
Dreams could also be an instant replay of the day’s events, charged with emotions and involving many jump cuts and strange juxtapositions. This is called the continuity hypothesis, which states that most dreams reflect the same concepts and concerns as our waking thoughts.
Dreams may spur creativity. In this theory, dreaming aids in problem-solving, coming up with solutions that may have eluded the dreamer’s waking life. In one famous instance, August Kekulé, a 19th-century German chemist, had a dream that featured the ouroboros (the mythical snake eating its tail), which inspired him to derive the ringlike composition of carbon atoms that make up the benzene molecule, a problem that he’d failed to solve in waking life. Dmitri Mendeleev arrived at his periodic table in a dream. “All the elements fell into place as required,” he recounted in his diary. “Awakening, I immediately wrote it down on a piece of paper.” Cosmologist Stephon Alexander dreamed about the role of symmetry in cosmic inflation. In contrast, Einstein’s dream, in which he saw cows jumping up and moving in a wavelike motion, led to one of the central tenets of his theory of relativity.
Dreams may have a culling function. It’s the brain’s way of “straightening up,” removing information that is not useful or incorrect. There is also a theory that dreams have no purpose whatsoever. Dreams could be incidental to sleep, a gratuitous process that contains the waste products of the day to which we may be tempted to impute importance that they don’t deserve.
With so much unknown about dreams and so many competing theories, dream experts in neuroscience and psychology continue their research with the possibility that they will never conclude why we dream.
Nonhuman Animal Sleep
All animals sleep. But do they dream? There is some evidence that they do, although obviously, they can’t tell us. If there are exceptions, they include dolphins and whales because underwater, only half of their brains are asleep at any given time. The behavior of animals while asleep—rapid eye movement, muscle twitching, and involuntary vocalizations—is similar to what humans observe while dreaming. A young elephant named Ndume, captured in Kenya after poachers killed his family, would wake up crying in the middle of the night, a response that Ndume’s handlers believed was equivalent to night terrors that traumatized humans would experience.
In the 2024 book When Animals Dream, the author David Peña-Guzmán, a professor of humanities and liberal studies, examines the evidence that strongly indicates that animals of all kinds dream and, further, that, like humans, they go through distinct sleep phases. (Bees have three; octopuses, two.) Research in the 1960s by the French neuroscientist Michel Jouvet on cats showed that their brain activity while asleep was no different from what would happen in the brains of cats in waking life when their prey was within striking distance. An octopus undergoes a series of color changes while asleep, leading some researchers to believe that these changes in their skin pigmentation may suggest that octopuses are dreaming.
Rats are known to “replay” movements in their sleep that they make while awake. Researchers reached this conclusion based on rats compelled to navigate mazes. While the rats were asleep, researchers observed that specific brain structures—maplike arrays of neurons—were activated, suggesting that the rats were dreaming about how they made their way through a maze while conscious. This behavior is hardly unique to rats. Neurological and electrophysiological measurements of the brains of various species have shown that they see, hear, and feel specific, identifiable scenarios in their sleep that appear to be direct replays of experiences they’ve had in waking life. Biologists studying zebra finches, an Australian species of bird that learns unique songs passed down from its family, found that in juveniles, the neurological patterns matched precisely, note for note, what they sang during the day. In other words, the birds were practicing in their dreams.
To dream experts, the existence of dreamlike states in animals raises the issue of what evolutionary purpose dreams might serve. Did dreaming arise among many species independently in the hundreds of millions of years that life has existed, or did it evolve once in some probably extinct creature and then persist as organisms branched out to follow their evolutionary trees? Some experts contend that the first creature to dream was a small, wormlike, sea-dwelling creature with no eyes and limited motion. We can’t imagine what use dreams might be for such a creature. Peña-Guzmán doesn’t take a position on whether such a creature was responsible for the first dreams or whether vertebrates (including us) and cephalopods like octopuses and cuttlefish can trace their capacity to dream back to a common ancestor.
Phases of Sleep
There are five phases of sleep in a sleep cycle. The first phase is characterized by light sleep, slow eye movement, and reduced activity. In the second phase, eye movement ceases and brain waves are slower, marked by occasional bursts of rapid waves called sleep spindles. In the third phase, delta waves appear—extremely slow waves interspersed with smaller, faster waves. In the fourth phase, delta waves predominate almost exclusively. This is called deep sleep because it is difficult to wake the sleeper up.
However, the fifth phase—rapid eye movement (REM) sleep—interests us because almost all dreaming occurs during the REM phase. REM sleep is characterized by rapid, irregular, and shallow breathing, eye jerking in all directions (that’s the rapid eye movement), and the temporary paralysis of the muscles. At the same time, the heart rate increases, blood pressure rises, and men can experience erections. Brain activity also increases during REM sleep, similar to wakefulness. Because brain activity intensifies compared to the non-REM phases, it may help explain the distinct types of dreams that are more vivid and fantastical. Dreams in other phases, by contrast, are typically more coherent and based on thoughts and memories that refer to a specific time and place. Most REM sleep, which cumulatively accounts for about two hours every night, takes place later in the sleep cycle, intensifying the closer we are to waking. Each dream lasts between 5 and 20 minutes.
“Dreams are mental imagery or activity that occur when you sleep,” explains Drerup, the sleep medicine expert at the Cleveland Clinic. “In REM sleep, we have less autonomic stability. Our heart rate increases. We don’t have the kind of steady, calm respiration that we do during other stages of non-REM sleep.” In all cases, researchers have to depend on first-person accounts of individuals who recall enough of their dreams to recount them. Babies and other mammals have to go by inference since they can’t yet see inside the brain to observe the actual process of dreaming. “That’s part of the reason why [dreams are] still kind of mysterious—they’re difficult to study,” Drerup says.
What Are Dreams?
Dreams appear to be a universal phenomenon, but what do they mean? Dream interpretation has a long history. It was practiced by the Sumerians, whose civilization flourished between 4100 and 1750 BCE, and Babylonians in the third millennium BCE. For these ancient civilizations, dreams held a religious significance. Dreams also enjoyed a prominent spiritual role in Ancient Egypt, where scribes would document them as long as the dreamer was important enough. (In the Old Testament, Joseph, who is sold into slavery by his brothers, subsequently wins the trust of the pharaoh because he proves able to interpret the ruler’s dreams.) For the Egyptians, dreams could have the force of commands or predictive power.
In the latter years of the 19th century, religion yielded to psychotherapy when it came to dream interpretation, most famously by Sigmund Freud, who perceived in dreams the expression of unconscious urges and wishes (especially regarding sex) that his patients wouldn’t dare consider, let alone act upon, in their waking life. Freud published his theories in his 1899 classic The Interpretation of Dreams. While Freud’s work has since come under fire and his interpretations debunked, his theories have been so well incorporated into our culture that they continue to influence how we interpret dreams.
Oneirology, the study of dreams, is no longer the realm of psychotherapists and has now been largely taken over by neuroscientists. Scientists, though, are still flummoxed by dreams. What is their function? Is there a discrete part of the brain responsible for generating dreams, or do the actions of several parts produce them? That they haven’t yet come up with a widely accepted hypothesis to explain the origin or purpose of dreams hasn’t discouraged them.
No cognitive state has been studied as extensively as dreaming. The dilemma scientists confront is the same as the Sumerians and Egyptians grappled with: how do dreams produce an experience that is often so vivid and emotionally charged that they may seem to hold significance that experiences in waking life usually do not? Subjective reports of dreams—which is all that scientists like psychotherapists have to go on—tend to be related to waking life, however bizarre and discursive, reflecting preoccupations, vexations, fantasies, memories, and fears that dominate the conscious mind, featuring appearances by individuals who are well-known to the dreamer but also those whom the dreamer may never have met.
