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Psychologists call physical activity that takes place in a natural environment green exercise. Within the first five minutes of any physical activity in nature, people report major shifts in mood and outlook. Importantly, they don’t just feel better—they feel different, somehow both distanced from the problems of everyday life and more connected to life itself. Taking a walk outdoors slows people’s internal clocks, leading to the perception of time expanding. Simply being in an environment with a variety of plant species increases people’s ability to gain perspective on their lives. Even just remembering a time spent in the presence of natural beauty makes people more likely to say they feel connected to the world around them, unburdened by everyday concerns, and in the presence of something greater than themselves.
Spending time outdoors can also calm an agitated mind. The emotions we are most likely to feel in nature—wonder, awe, curiosity, hope—are natural antidotes to worry, distraction, and depression. As one man describes the internal state he felt while canoeing through Canada’s wild rivers, “There were no sharp things inside me . . . not a sense that something was wrong, or I needed to work on something, there was just a peace, there was tranquility, there was acceptance, there was harmony.” Others report finding in nature “a complete sense of belonging” and a feeling of being held, “similar to when you really genuinely hug a person.” The psychological benefits of outdoor activity can be profound. At the Hong-reung Arboretum in Seoul, Korea, middle-aged adults being treated for depression walked among the trees and alpine plants before participating in their weekly cognitive behavioral therapy sessions. At the end of one month, 61 percent of the forest-walkers were in remission, three times the rate of patients whose psychotherapy took place in a hospital. In an Austrian study, adding mountain hiking to standard medical treatment reduced suicidal thinking and hopelessness among individuals who had previously attempted suicide.
Our tendency to spend most of our days indoors is a relatively recent reversal. The human brain evolved over a long period of history when humans spent most of their time outdoors, interacting with the natural world. Because of this, the human mind responds to nature in ways that bring out many of our cognitive strengths. Being active outdoors can help us tap into the human capacity for mindfulness, as well as the transcendence of being connected to something bigger than ourselves. It puts us in touch with the innate joy that biologist E. O. Wilson calls biophilia, or love of all that is living. It helps us see our own lives from a broader point of view. Understanding how green exercise achieves these effects can teach us something important about the human mind—both how it can get stuck in a cycle of suffering and how we can find peace.
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One cold February afternoon, writer Maura Kelly left her Brooklyn apartment, not sure where she was going, but knowing she had to get out. Kelly had lived with depression for years, but her despair had been escalating as one thing after another fell apart: a relationship ending, a string of professional disappointments, a sleep disorder that left her exhausted. The day she fled her apartment, Kelly was trying to escape the prison her mind had become and the thoughts that tormented her: I’m not good enough. I will never be happy. I’ll always be alone. All my effort will come to nothing.
She found herself walking the sloping hills of Fort Greene Park, a thirty-acre urban forest filled with Norway maples, American elms, and Austrian pines. She was the only person braving the bitter cold. Among the trees and fresh air, Kelly felt something shift. As she would later write in an essay, “I felt more free, less trapped in my apartment and in my head. . . . instead of being taunted into one dark corner after another by the voices in my head, I went one way, round and round, and turtled deep in my jacket, felt better able to listen to what was good in myself.” Inside her apartment, she had felt completely immersed in her negative thoughts. It was as if her worries and self-judgments filled the space around her and became the very air she was breathing. “Outside,” she told me, “I was around more positive thoughts. Like, ‘Ah, the big sky, the big trees, the air—this is good. I’m okay. I’m still alive. I’m free.’”
Unlike a runner’s high, the mind-altering effects of green exercise kick in almost immediately. These fast-arriving benefits cannot be explained by the slow accrual of feel-good chemicals like endocannabinoids or endorphins. Instead, it’s as if being in nature flips a switch in the brain to transport you into a different state of mind. The question is, what switch? If neuroscientists had been able to observe what was happening in Maura Kelly’s brain as she walked through Fort Greene Park, they almost certainly would have detected a change in the default mode network. This brain network was first identified twenty years ago, when researchers used functional neuroimaging to document the baseline state of an awake human brain. Before then, brain-imaging studies had focused on figuring out which brain structures are active during specific tasks. Neuroscientists would scan people’s brains while asking them to solve math problems, memorize word lists, or analyze the emotions expressed in photographs. Eventually some of these researchers thought to ask: What happens in the brain when a person is lying in a brain-imaging machine waiting for instructions? What does the brain do when you’re not doing anything and the mind is allowed to wander?
When neuroscientists analyzed the brain’s baseline state, the answer surprised them. The resting brain turns out not to be resting at all. Many systems throughout the brain are active, including those linked to memory, language, emotion, mental imagery, and reasoning. Even more stunning was the finding that all human brains, at rest, slip into a similar state. Neuroscientists dubbed this pattern of brain activity the default. Left to its own devices, the human mind holds imaginary conversations, replays past experiences, and reflects on the future. It especially likes to think about you, your goals in life, and your relationships with others. This default state is essential for functioning in a social world. The brain’s baseline activity is also how we remember who we are. Its inner chatter and imagery provides the awareness that you exist as a specific individual, with preferences, aspirations, and problems, one who continues across time and context. You wouldn’t want the alternative—a brain that struggles to create a coherent sense of self or place in the world. That is what happens in late-stage Alzheimer’s disease, when pathological tangles ravage the core structures of the default network.
However, the default state also has a downside. For many of us, the mind’s default has a negative bias. Its most familiar habits are to ruminate on past hurts, criticize ourselves or others, and rehearse reasons to worry. The default state can also become a mental trap. In theory, when you focus on something—a conversation, a movie, work—the default mode quiets down and allows the brain to enter a state of outwardly directed attention. But people who suffer from depression or anxiety don’t make this switch as easily. They show unusually high activity in the default mode network, and they get stuck in the default state, making it difficult to focus on anything or anyone else, or even to fall asleep. For some, the mind can even become addicted to rumination. The brain’s reward system—not a core part of the default mode network—can become highly connected to the structures within the default mode network that are associated with memories, worry, and thinking about yourself. Every time you revisit a familiar fear or judgment, the reward system says, “Yes, more of that!” It’s as though the brain is convinced that some good might come from the rehashing, worrying, or self-criticism. When this happens, you can find yourself as unable to escape these mental habits as a heroin user trying to suppress their cravings and compulsions.
One of the most effective ways to quiet the default state is meditation. In brain-imaging studies, focused breathing, mindfulness, and repeating a mantra have all been shown to deactivate hubs of the default mode network. In one unusual case study, neuroscientists at the Weizmann Institute of Science in Israel studied the brain activ
ity of a sixty-four-year-old meditation master who had spent more than twenty thousand hours meditating. As he moved through different states of consciousness, from a typical default state to a “selfless” state of pure awareness, his brain scan showed the default mode network disengaging.
Green exercise appears to do something similar to the brain, but without the need for such dedicated mind-training. At least one study has tried to capture this effect with brain-imaging technology. Researchers at Stanford University sent participants out on a ninety-minute walk. Some hiked the Dish, a scenic trail in the foothills near campus, while others walked along one of the busiest streets in Silicon Valley. Before and after the walk, neuroscientists put the participants in an fMRI machine to capture their brains’ resting activity. The participants also answered questions about their state of mind, including how much they agreed with statements like “My attention is focused on aspects of myself I wish I’d stop thinking about.” After the scenic hike—but not the walk on the busy roadway—participants reported less anxiety and negative self-focused thinking. Their post-walk brain scans revealed less activity in the subgenual cortex, an area linked to self-criticism, sadness, and rumination. Individuals who suffer from depression show more activity in this part of the brain during “rest” than people who are not depressed. A walk in nature selectively silenced this part of the default state’s stream of consciousness.
Notably, the very same neurological change has also been detected in two of the most promising experimental treatments for depression: transcranial magnetic stimulation, which delivers an electric current to the brain through a magnetic coil on the scalp, and ketamine, an anesthetic used on the battlefield during the Vietnam War and popularized as a recreational drug during 1990s rave culture. Magnetically stimulating the prefrontal cortex reduces the active connections between the subgenual cortex and other brain regions in the default mode network. This change is linked to reduced symptoms of depression among patients who had not responded to any previous antidepressant medications. Intravenous infusions of the drug ketamine disrupts the brain’s resting state in precisely the same way. This reorganization of the default mode network, which persists twenty-four hours after being given a dose of ketamine, overlaps with the peak antidepressant effect of the drug.
There’s no evidence that these three “therapies”—transcranial magnetic stimulation, ketamine, or exercising outdoors—are interchangeable, and no responsible scientist or health-care provider would encourage someone who is clinically depressed to forgo medical treatment in favor of a hike. Still, it is fascinating that the short-term neurological effect of a walk in a park—something accessible to many people—so closely resembles the mechanism of two cutting-edge treatments for depression. This may explain why the psychological benefits of being in nature are most pronounced among those who struggle with depression. It is also a reminder that making time for physical activity is not self-indulgent. For many, it is an act of self-care, even self-preservation. In his memoir Dip, photographer Andrew Fusek Peters—whose father died by suicide—explains how swimming in the rivers, lakes, and waterfalls of Shropshire and Wales interrupted the usual “thought-torture” that defined his own severe depression: “Diving into wild water is the great bringer-back of reality. A perfect present tense, a right-here, right-now moment. The senses are so filled by the trees, the light, the sound of birds, of shivering leaves, the fierce, squeezing clinch of water—there’s no space for thought shadows.” Psychotherapy helps him challenge his most self-destructive thoughts, but “it takes an awful lot of cognitive behavior therapy to change the tape.” Swimming outdoors turns the tape off.
Peters’s description of what he experiences in the water captures something important about how nature affects the mind. When you are absorbed in your natural surroundings, the brain shifts into a state called soft fascination. It is a state of heightened present-moment awareness. Brain systems linked to language and memory become less active, while regions that process sensory information become more engaged. The senses are heightened and inner chatter quiets. This shift can be a tremendous relief for people who struggle with anxiety, depression, and rumination, for whom the default mode is relentlessly verbal, generating words and phrases that echo in their minds. By flooding the senses with pleasant stimulation, nature draws your attention outward and interrupts the linguistic assault. There is room for curiosity about and appreciation for the world around you.
Mindfulness practices teach people how to access this state of heightened sensory awareness intentionally. Brain-imaging studies reveal that after people receive mindfulness training, their resting state shifts away from rumination and toward present-moment attention, even while lying in an fMRI machine. Among highly experienced meditators, mindfulness may even replace the usual default mode to become a new baseline state of mind. Meditation teachers like to say that this present-moment focus, including the happiness that comes from it, is the natural state of the mind. To those who live with anxiety or depression, such a claim can seem preposterous. And yet as I learned more about how nature affects the brain, I began to wonder if the human mind has two distinct default modes. There is, of course, the default mode neuroscientists observe when participants are trapped inside a functional imaging machine, a state defined by mind-wandering, self-reflection, and rumination. Might there also be a very different default mode that reveals itself when we are in nature?
Alexandra Rosati, a psychologist who studies the evolutionary origins of the human mind, points out that two pressures shaped the development of the human brain. The first was our need to cooperate in small groups. This pressure gave rise to social cognition, our ability to think about other people. This includes our tendency to define ourselves in relationship to other people and to reflect on our position within our tribe. The second pressure on human evolution was our need to cooperate with the natural environment to find food. According to Rosati, this need gave rise to mental skills she labels foraging cognition. Just as natural selection favored anatomical changes—like longer legs and powerful gluteal muscles—that helped humans hunt and gather, it also reinforced mental abilities that helped our ancestors find what they needed. Humans developed a fine-tuned spatial awareness, a mindset open to discovering the possibilities around us, and the patience to continue searching.
As I read Rosati’s work, I started to think about how the typical default state is essentially a way to practice the social cognition skills humans developed to thrive in groups. In contrast, the alternate default state of mindfulness—one of open awareness to the environment, with a sense of curiosity and hope—maps onto the “mind” Rosati describes as foraging cognition. Neuroscientists argue that the typical default state exists because humans need to rehearse our social selves to survive as a social species. If this is the case, why wouldn’t we also have a default state of mind that reflects our need to engage with nature? For our early ancestors, the ability to explore and find resources in the natural world was as key to their survival as the willingness to share. Surely the human brain evolved to reflect this need as much as it has been shaped by our dependence on one another.
Perhaps the context we find ourselves in determines which default state the mind reverts to. Humans who find themselves disengaged from natural environments may come to know primarily the self-focused default state. Spending time not just indoors, but also on social media, pushes us toward social cognition and, often, rumination. Without regular time spent outdoors, we can lose touch with the default state of open awareness. By reconnecting with nature, we refamiliarize ourselves with this other aspect of what it means to be human. This is a big part of what draws people to green exercise. Outdoors, it is possible to rediscover a self that is not solely defined by your roles and relationships with others, or by your past. You are free to be a self that is in motion, attuned to the present moment, and open to what the world has to offer.
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The psychological effects of physical activity are often compared to mind-altering substances. The runner’s high mimics a mild cannabis buzz. Synchronized dancing produces a glow not unlike ecstasy. Moving to music provides an adrenaline rush similar to stimulants. I’ve even heard a good yoga stretch described as turning one’s blood into wine. These comparisons, while imperfect, provide a useful framework for understanding both the appeal and benefits of different kinds of movement. When I began to consider whether such a parallel exists for green exercise, my first thought was that it would have to be some kind of pharmaceutical aimed at anxiety or depression. But if you focus on what is unique about green exercise, the class of drugs it most closely resembles is the entheogen, a category that includes psilocybin, ayahuasca, and LSD. Entheogens are plant-derived substances described by advocates as consciousness expanding, and are ingested to induce a religious or spiritual experience. Like green exercise, these drugs alter consciousness by temporarily reorganizing the default state. During an LSD trip, changes in default network connectivity correlate with the user’s feelings of oneness with the universe.
Many people report life-changing insights and moments of self-transcendence while under the influence of these drugs, an outcome that is also surprisingly common for spending time in nature. Eighteen percent of the people in the United States say they have had an intense spiritual experience while in nature, and almost half of all mystical experiences take place in a natural setting. The most common of these experiences is the unity sensation, a feeling of union with something bigger than oneself, accompanied by a wave of love and sense of deep harmony. In Finding Ultra, Rich Roll recalls such an experience as he ran through the hills of California’s Topanga State Park. “I didn’t just feel amazing. I felt free. . . . For the first time in my life, I felt that sense of ‘oneness’ I’d only previously read about in spiritual texts.” The unity sensation is often felt as a union with nature itself. One woman, age fifty, experienced just such a diffusion of self while hiking the Grand Canyon in Arizona. “I felt a complete merging with the surrounding environment. Instead of sitting back and observing it . . . it’s like I was moving into it in some way, or rather it was moving into me . . . I suppose what I experienced was transcendence, losing myself into my surroundings. It was expansive and at first I was afraid and then deeply comforted and filled with a sense of complete peace.”
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