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Just 20 minutes in an urban park helps lower stress-hormone levels, study finds

Lakes of the Isles walk
MinnPost file photo by Steve Date
The study found that extending the time spent in nature beyond 30 minutes doesn’t have much additional benefit.

As little as 20 minutes of sitting or strolling through a city park — or any outdoor place that makes you feel more connected to nature — helps reduce the levels of stress hormones circulating in the body and thus may help reduce stress itself, according to a small but interesting study published Thursday in the journal Frontiers in Psychology.

Many Americans are stressed out — almost half of all adults, according to a 2018 Gallup Poll. That is a worrisome figure, as chronic stress has been linked to a variety of health concerns, including anxiety, depression, headaches, digestive problems, sleep problems, high blood pressure and heart disease.

Plenty of previous research has shown that contact with nature helps to mollify the body’s stress response and ease stress-related symptoms. For this reason, some health care practitioners, both in the United States and in Europe, have begun to write nature prescriptions — sometimes called “nature pills” — for their patients’ mental and physical well-being.

What this new research suggests is that you don’t have to devote a full day, or even several hours, to rambling through a park or other green space to benefit from that prescription. In fact, the study found that extending the time spent in nature beyond 30 minutes doesn’t have much additional benefit.

“We know that spending time in nature reduces stress, but until now it was unclear how much is enough, how often to do it, or even what kind of nature experience will benefit us,” says MaryCarol Hunter, the study’s lead author, in a released statement. “Our study shows that for the greatest payoff, in terms of efficiently lowering levels of the stress hormone cortisol, you should spend 20 to 30 minutes sitting or walking in a place that provides you with a sense of nature.”

Hunter is a landscape architect and ecologist whose research focuses on the social, psychological and ecological aspects of urban design.

Innovative study design

For the study, Hunter and her colleagues recruited 36 residents of Ann Arbor, Michigan, the location of the University of Michigan’s main campus. The recruits ranged in age from 22 to 68. Almost all were women (33) and white (31).

The researchers instructed the study’s participants to spend 10 minutes or more, at least three times a week, in an outdoor space that made them feel as if they were interacting with nature. They were to do this for eight weeks during the summer months of June and July.

Because they wanted the study to reflect how people might realistically engage with nature as they go about their everyday lives, the researchers gave the participants permission to choose where and when they visited those outdoor spaces.

To minimize factors known to influence stress, the participants were instructed to visit the outdoor space during daylight. They could sit or walk in the space, but were to avoid aerobic exercise. They were also instructed to avoid accessing the internet, taking phone calls, reading or engaging in conversations while there.

In other words, they were simply to commune with nature.

“Building personal flexibility into the experiment allowed us to identify the optimal duration of a nature pill, no matter when or where it is taken, and under the normal circumstances of modern life, with its unpredictability and hectic scheduling,” said Hunter.

Key findings

At four times during the study — every two weeks — the participants collected saliva samples before and after their nature experience. The researchers used those samples to measure levels of the stress hormone cortisol and of another body chemical known to be related to stress, the enzyme alpha-amylase. The measurements were adjusted for the time of the day they were collected, as levels of both cortisol and alpha-amylase rise and fall during each 24-hour cycle.

The study found that the participants’ levels of cortisol dropped by an average of 21 percent after spending time in an urban green space. Levels of alpha-amylase also fell, by an average of 28 percent — but only when the participants spent most of their nature break sitting rather than walking.

The effects on cortisol were most pronounced when the nature experience was between 20 and 30 minutes long. Levels of the hormone dropped further when participants spent longer periods in a green space, but at a slower rate.

Limitations and implications

This study was quite small, and its participants were not demographically diverse. So the findings might not be applicable to broader groups of people.

Furthermore, the study measured only drops in biomarkers (levels of cortisol and alpha-amylase) for stress. It didn’t measure whether the 20- to 30-minute nature experiences actually reduced the participants’ feelings of stress — or if those experiences had any direct impact on specific health outcomes, such as lowering blood pressure.

Still, the study’s findings are in sync with a growing body of research that supports the idea that spending time in and/or living near a green space can have a positive effect on health.

The findings may also help with the prescribing of “nature pills,” says Hunter.

“Health care practitioners can use our results as an evidence-based rule of thumb on what to put in a nature-pill prescription,” she says. “It provides the first estimates of how nature experiences impact stress levels in the context of normal daily life.”

FMI: You can download and read the study in full at the Frontiers in Psychology website.

Comments (2)

  1. Submitted by JoAnn Meyer on 04/04/2019 - 11:33 am.

    Read the history of pocket parks in Paris at the beginning of the Industrial Revolution and you will learn this is not new information. Not certain it is necessary to spend time and money on researching what we have known for a long time.

  2. Submitted by David Motzenbecker on 04/08/2019 - 11:25 am.

    Hi Susan!

    I just wanted to thank you for writing the piece highlighting the recent Michigan study on time in nature/stress reduction.
    I couldn’t agree more with the general message of the study, but find some issue with the point that ALL it takes is 20 minutes.
    The reason why we’re so frazzled these days is because as a species we’re systematically and exponentially disconnecting ourselves from nature.

    I, too, am a licensed landscape architect who has been practicing for nearly 20 years. For the past decade I’ve been focused specifically on biophilic and salutogenic design, as well as Shinrin Yoku and it’s benefits to the human system.

    After spending nearly all of that decade trying to get clients to understand and buy into the reasoning, scientific evidence was scarce and not many were interested. Recently, more evidence has come to light to support time in nature with hard metrics – such as the boost in cancer-killing white blood cells after a couple hours in the woods by nearly 50%.

    I believe in this passionately. I got trained and certified as a Forest Therapy Guide through the only global certifying organization – Association of Nature and Forest Therapy Guides (ANFT). You can find more info here: http://www.motzstudios.com.

    Perhaps I can offer you a 2 hour, immersive, intentional, and meditative forest bathing walk as an comparison to the short 20 minute burst of nature.

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