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.

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“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.

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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.