What causes illness? At one level, the answers are quite varied — viruses, bacteria, stress, poor nutrition, lack of exercise, poor immune function and many more.

But from a wider perspective, I believe that a great deal of modern illness, both physical and mental, is due to a single cause that biologists call evolutionary mismatch. This means that a human or animal that evolved helpful traits in a given environment can find those same traits unhealthful — even deadly — in a radically different environment.

Fortunately, if we understand this, we can try to spend time in environments that match the mental and physical tendencies we retain from our distant ancestors.

Which brings us to forest therapy.

The concept is simple. Since most of our evolution happened in green, wild places instead of modern cities full of buildings, cars and computer screens, spending time in the woods agrees with our ancient mental and sensory circuitry — and so can make us happier and healthier.

Of course, anyone can head off into the woods independently to reap this benefit. An intuitive sense that we need wilderness is probably what sends 37 million American households camping each year. But a formal therapeutic practice of regaining health in the forest — complete with studies to measure health effects — began in Japan in the 1980s. The practice there is called shinrin-yoku, which roughly translates to “taking in the forest atmosphere” or “forest bathing.”

Research suggests that benefits include:
- Boosted immune system functioning, with an increase in the count of the body’s m natural killer (NK) cells
- Reduced blood pressure
- Reduced stress
- Improved mood
- Increased ability to focus, even in
- children with ADHD
- Accelerated recovery from surgery or illness
- Increased energy levels
- Improved sleep

Achieving these need not take long — one study showed looking at forest scenery for just 20 minutes significantly lowered saliva concentrations of the stress hormone cortisol.

Organized forest therapy has come to this country through the U.S. Association of Nature and Forest Therapy Guides and Programs, founded in 2012. The group trains forest therapy guides, and is establishing programs nationwide.

A typical session in the U.S. lasts roughly three hours, but covers no more than a mile. This makes the experience accessible to a wide range of people. Those who are not physically fit are encouraged to go slowly and breathe deeply.

Of course, forests aren’t the only natural environments. Gazing at the ocean — no doubt an ancient human practice — appears to offer healing power as well.

A study at the University of Exeter looked at how natural water environments can boost physical and mental health. Researchers examined census data in England and found that people who lived closer to the coast reported better health. This might seem like a “wealth effect” of the homes of rich people — who tend to be healthier than poor ones — clustering at the seashore. But the study found that health benefits were greatest for poor communities near the beach, especially compared to equally poor communities inland.

Beyond forests and oceans, the list of healing natural environments is endless. Personally, I find the desert uplifting. You can benefit from nature therapy in many places.

In my book, Spontaneous Happiness, I wrote at length about the importance of regularly spending time in natural environments — and resisting the urge to bring our glowing, beeping, nagging gadgets along. Biologist E.O. Wilson coined the term biophilia, which means “love of life or living systems.” It’s a lovely word that describes an innate human need, one that I’m sure is as real as our needs for love, food, sex and community.

The right way to let nature into your life depends upon you. People who like groups and guidance may find an organized forest therapy experience appropriate. If you are among them, visit natureandforesttherapy.org and click the “Get Started” tab.

If you are more of an independent sort, be creative. Those recovering from extreme stress may need two weeks in a tent in the woods. For others, the simple act of taking a long walk outdoors after dinner rather than staring at a TV screen can work wonders. Just let nature into your life in any way that works for you.