According to scientific research, spending 36 hours in the woods per month provides the maximum dose of an immunity boost and stress reduction thanks to things like:
the wavelength of the color green, which soothes the nervous system
terpene molecules wafting from the pine needles, which help to expand the lungs
the unpaved ground, which promotes balance and improved proprioception
However, I'd argue that there is a simpler explanation for why we feel good in the woods, (or at least why I do) which is that it's an environment free of performance expectations. There are no clocks and no screens. No email, no notifications, no social media humble-bragging. Assuming you're not a hard-core climber or mountain biker, there is not much you can do in the woods that would qualify as a performance or an achievement.
And that is pretty much the point of forest bathing. Shinrin-yoku, as it's known in Japan, is the practice of going to the woods, turning off your city/work/media filters, and turning on your senses to connect with nature.
My garden therapy excursion last month consisted of a weekend of German forest bathing. In the Grunewald woods of Berlin, we were always close to a trail and occasionally even within earshot of the Autobahn, yet we were still far enough removed from the city to need a full day's supply of food and water in our backpacks. Each day, we had guided meditations and sensory exercises like sniffing the soil and handling the surface of pine cones with our eyes closed.
But the highlight assignment for me was stringing up a hammock between the trees and taking an hour-long nap after lunch each day. It stands out for me as an exercise because I found it both appealing and daunting. One part of me kept wanting to ask, "Are we really supposed to go to sleep?". Also, while I like the idea of napping, I usually find it near-impossible to slip under the surface of consciousness during daylight hours. However, being suspended in my little cocoon under the branches must have activated some kind of ancient primate memory, because my light-bulb of consciousness got switched off right away.
And the whole experience was reinvigorating. Although I was near-exhausted from a hectic week at work going into the weekend, and we walked considerable distances with our heavy backpacks, I felt much less tired by the end.
So there's my personal testimonial for forest bathing — with one important caveat. The idea of going into the woods to recharge one's batteries, and getting all of the health benefits now proven by the scientific research I mentioned at the beginning, risks falling into the default mode of consumerism/extractive thinking where nature consists of so many "ecosystem services" that are just there for human convenience, rather than a living world of community, intelligence and relationships.
When we go into the forest, we don't just have to grab all we can get "for free". We can give something back. At the very least, we can offer some words of gratitude. We could pour some water on the ground, and leave some of our food for the other beings, as my group did in the places we visited.
And finally, we can return the favor: if we are freed of the constant pressure to produce some kind of measurable benefit when we are in nature, then maybe we can allow nature to not necessarily do anything for us as well. We could allow the forest to simply be. And then forest bathing could become an exercise in simply being together: we and the trees.