The modern environment, with its cacophony of traffic noises, overcrowding, and artificial lights, teems with potential stressors. Chronic exposure to such stressors may heighten the body’s sympathetic nervous system and negatively affect health, leading to hypertension, mental health issues, and impaired immunity. Natural environments, on the other hand, are believed to decrease stress, but there is no systematic review addressing the effect of nature on the activity of the sympathetic nervous system.
This was a systematic review of 12 studies (8 cross-sectional studies and 4 randomized controlled trials, with an average of 760 participants per trial) summarizing the effect of nature exposure on physiologic markers and perceived levels of stress. In five studies, the primary outcome was perceived stress using a validated scale, while six studies used physiological markers of stress as the primary outcome (e.g. fMRI scans, salivary cortisol, blood pressure measurements, heart rate and rhythm, muscle tension) and one used both.
Overall, increased exposure to nature was associated with improvements in physiologic stress markers and perceived stress levels. For example, the cortisol levels of people living in neighborhoods with more green space were lower than the levels of people living in areas with little green space. In fact, as the percentage of green space increased, reported levels of stress decreased. Exposure to nature for thirty minutes over the course of a week reduced blood pressure in people with hypertension, and a 90-minute nature walk resulted in greater reductions of brain activity in regions involved with sadness and negative emotions, compared to a walk in an urban environment.
Moreover, after participants were exposed to a psychological stressor, listening to nature sounds or looking at a nature picture reduced their sympathetic activity more quickly than listening to or looking at non-natural sounds and pictures. However, there was one twin study in which increased access to green space was only associated with a decrease in depression, and not stress.
The concept of nature as a treatment for stress is one rationale behind the practice of “forest bathing” (mindfully spending time in nature). As noted in the August edition of Study Summaries, a recent review study on forest bathing for hypertension found that forest bathing effectively reduced blood pressure, heart rate, and physiological and psychological stress.
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This Study Summary was published on December 7, 2020.
- ^Katherine Ka-Yin Yau, Alice Yuen LokeEffects of forest bathing on pre-hypertensive and hypertensive adults: a review of the literatureEnviron Health Prev Med.(2020 Jun 22)