A large and rapidly expanding research literature links exposure to natural environments (e.g., forests, parks, gardens, “greener” neighborhoods) with benefits across mental health, cognitive function, cardiometabolic risk, sleep, and—under some conditions—immune function. Evidence comes from experimental studies (including forest-vs-urban comparisons), observational cohorts using greenness indices (e.g., NDVI), and emerging natural experiments and trials. Overall, findings are most consistent for stress physiology and affective outcomes, with growing support for cognitive and brain-based endpoints and for longer-term psychiatric and cardiovascular outcomes. However, key limitations remain: heterogeneity in how “nature exposure” is measured, inconsistent reporting of dose (frequency × duration × quality), and uneven representation of children, adolescents, and historically marginalized populations. This article synthesizes the evidence, clarifies mechanisms, and translates research into practical guidance for Forest Bathing Guides and policymakers.

1. Why “nature exposure” became a health variable

Modern life changed the baseline conditions under which human nervous systems evolved: dense urbanicity, chronic attentional demand, noise, heat, and air pollution. The public-health question is no longer whether nature is “nice,” but whether it is measurably protective—and under what conditions.

Two classic frameworks still organize much of this research:

Below 

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Table 1. Comparison of Core Theoretical Frameworks Explaining Nature–Health Associations

Theory	Primary Proponents	Core Assumption	Primary System Affected	Key Mechanism	Typical Outcomes Observed	Type of Supporting Evidence
Stress Reduction Theory (SRT)	Roger Ulrich	Humans possess an evolutionarily shaped positive affective response to natural environments that facilitates rapid stress recovery.	Autonomic nervous system; endocrine stress pathways (HPA axis)	Downshifting of sympathetic activation and enhancement of parasympathetic tone in low-threat natural settings	↓ heart rate; ↓ blood pressure; ↓ perceived stress; often ↓ cortisol; improved affective state	Experimental forest vs. urban comparisons; psychophysiological field studies; laboratory exposure paradigms
Attention Restoration Theory (ART)	Rachel Kaplan & Stephen Kaplan	Directed attention becomes fatigued in cognitively demanding environments and can be restored through exposure to natural settings characterized by “soft fascination.”	Executive attention networks; cognitive control systems	Reduced cognitive load combined with involuntary attention engagement (“soft fascination”), enabling restoration of directed attention capacity	↑ attention performance; ↓ mental fatigue; ↑ perceived restorativeness; improved cognitive clarity	Experimental attention-task studies; field experiments; self-report restorativeness measures

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2. How scientists measure “nature exposure”

One reason findings can look inconsistent is that “nature exposure” is operationalized in different ways:

         Quantity of greenness near home/school: NDVI, tree canopy, land-cover classifications
         Access/proximity: distance to parks/forests, density of green spaces. 
         Actual time in nature (“dose”): minutes per week, visit frequency, GPS-based tracking 
        Quality: biodiversity, maintenance, perceived safety, amenities, soundscape

A widely cited dose study suggests that ~120 minutes/week of recreational nature contact is associated with higher likelihood of reporting good health and well-being, with benefits rising up to a point (often reported around ~200–300 minutes/week) before plateauing.

3. Summary of evidence across health outcomes

Table 2. Nature exposure and health outcomes: what the evidence most consistently supports

4. Experimental evidence: what happens after brief exposure?

Experimental studies matter because they manipulate the environment (nature vs. urban) and observe what changes after exposure. That design reduces (though never fully eliminates) the “self-selection” problem: people who already feel well may be more likely to spend time in green spaces. What these studies show, again and again, is an acute “state shift”: within minutes to a couple of hours, many people move toward a profile consistent with lower stress load, improved affect regulation, and restored attention, especially when the comparison condition is traffic-dense, noisy, or visually complex urban space.

4.1. Rumination and depression-relevant circuitry

One of the most influential demonstrations comes from a study comparing a 90-minute walk in a natural setting vs. an urban setting. Participants in the nature condition reported reduced rumination (repetitive, self-focused negative thinking), and brain activity changed in a region often implicated in depression-linked thought patterns (subgenual prefrontal cortex). In plain language: nature did not just “feel better,” it shifted a cognitive-emotional loop that predicts poorer mental health.

Why this is important for Shinrin-Yoku practice:

• Rumination is not simply “thinking too much.” It is often sticky, involuntary, and physiologically activating.

• The finding supports what guides often observe: once the mind stops recycling a problem story, the body can soften, perception widens, and participants become more available to sensory contact and meaning-making.

Practical translation into session design:

• Start with external anchoring (sound, texture, temperature, light) before deep reflection invitations.

• Keep walking pace gentle and allow attention to “land” repeatedly (short stops help).

• Avoid early invitations that intensify self-evaluation (e.g., “What’s wrong with your life?”) until the nervous system has downshifted.

4.2. Autonomic and endocrine stress markers

A large proportion of experimental “forest vs. city” research measures autonomic physiology: what the nervous system is doing in real time.

Common findings after brief forest exposure (often 15–60 minutes, sometimes longer):

• Lower systolic/diastolic blood pressure

• Lower pulse rate

• Shifts in autonomic balance consistent with increased parasympathetic activity (rest-and-digest) and/or reduced sympathetic drive (fight-or-flight)

• Improved self-reported calm, comfort, and vitality

This pattern fits a core idea: nature exposure can act like a down-regulating context—especially when sensory load is softer and perceived threat is lower.

The cortisol nuance (why results sometimes look “messy”)

Cortisol is popular because it is a biological stress marker, but it’s also a finicky one:

• It follows a strong diurnal rhythm (high in the morning, declining across the day).

• It is sensitive to food intake, caffeine, exercise intensity, sleep, menstrual cycle, medications, and even the stress of being measured.

• “Before vs. after” sampling without strict timing control can produce mixed outcomes even when participants clearly feel calmer.

So when cortisol doesn’t change, it doesn’t automatically mean “nature didn’t work.” It may mean the measurement design wasn’t tight enough, or the outcome window was too short, or the participant started at a low baseline.

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Table 3. What brief-exposure experiments typically measure—and what to watch for

Experimental focus	Typical “brief exposure” design	Key outcome measures	Usual direction after nature exposure (vs. urban)	Methodological pitfalls (why findings vary)	What this means for guides
Rumination & depression-linked processing	60–90 min walk in nature vs. urban; sometimes with questionnaires + neuroimaging	Rumination scales; fMRI activity in emotion-regulation networks	↓ rumination; neural activity shifts consistent with reduced maladaptive self-focus	Small samples; individual differences (baseline stress/trait rumination); setting quality	Use early invitations that externalize attention and reduce self-judgment; allow gentle movement + pauses
Autonomic regulation	15–60 min sitting/walking; nature vs. city; sometimes cross-over	HR, BP; HRV; sympathetic/parasympathetic indices	↓ HR, ↓ BP; HRV often improves	Temperature, noise, terrain, pace; talking/social interaction; caffeine; prior exertion	Pace the session for physiological settling: slower transitions, quieter stops, fewer instructions once settled
Endocrine stress (cortisol)	Pre/post saliva cortisol around nature vs. urban exposure	Salivary cortisol	Often ↓ cortisol, but not always	Diurnal rhythm; food/caffeine; sampling timing; baseline variability; lab/field constraints	Don’t “sell” cortisol as guaranteed. If measuring, standardize time-of-day and pre-session behaviors
Subjective stress & affect	Nature vs. urban exposure; questionnaires before/after	Perceived stress; mood/affect scales; restorativeness	↓ perceived stress; ↑ positive affect; ↓ negative affect	Expectancy effects; novelty; group dynamics; weather	Pair physiological pacing with simple language—people often report benefits even when biomarkers are mixed
Cognitive restoration	Short exposure + attention tasks	Attention/executive function tasks; perceived restorativeness	↑ attention performance; ↑ restorativeness	Practice effects; task choice; distractions; insufficient “dose”	Build in quiet absorption periods; reduce cognitive demand (less teaching, more sensing)

5. Observational evidence: what about long-term mental health and equity?

If experimental studies show us what happens after a walk, observational research asks a different question: What happens when people live for years in greener—or less green—environments?

These studies use large population datasets, sometimes tracking hundreds of thousands (or even millions) of individuals across time. They cannot randomize people to neighborhoods, but they can examine patterns that unfold across years, including:

• Incidence of depression
• Psychiatric diagnoses
• Cardiovascular disease
• All-cause mortality

• Childhood developmental outcomes

Because they capture real-world living conditions, observational studies are crucial for public health and urban policy.

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5.1. Depression and psychiatric outcomes across the life course

The 2021 narrative review synthesizing recent literature reports a consistent pattern:
Higher residential greenness is associated with lower risk of depressive symptoms and certain psychiatric outcomes, especially in longitudinal studies where exposure precedes diagnosis.

Key themes emerging from this body of work:

• Childhood exposure matters. Several large cohort studies suggest that sustained exposure to green space during childhood is associated with lower risk of psychiatric disorders later in life. This supports a life-course perspective: early environmental conditions may shape stress reactivity, cognitive development, and emotional regulation.

• Urban density modifies effects. Protective associations are often stronger in densely populated urban environments, where green space may represent a greater contrast to baseline stressors (noise, crowding, heat, pollution).

• Dose and continuity matter. It is not only whether green space is present, but whether exposure is sustained over time.

However, variability across age groups, countries, and measurement approaches reminds us that greenness is not a uniform exposure—it interacts with culture, safety, walkability, and social cohesion.

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5.2. Meta-analytic synthesis: depression and anxiety

A 2023 meta-analysis integrating multiple studies strengthens the case that higher green space exposure is associated with lower depression and anxiety outcomes.

Meta-analyses are important because they:

• Pool results across many independent samples

• Increase statistical power

• Identify consistent patterns despite methodological differences

The overall signal supports green space as a population-level preventive factor rather than only an individual-level wellness tool. In other words: Green space is not just therapeutic. It may be infrastructural prevention. This reframes nature not as luxury, but as part of the public health system.

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5.3. Equity and the “equigenic” effect

One of the most important developments in the field concerns health equity.

A 2024 PRISMA-based review focusing on disadvantaged groups suggests that green space exposure can have protective mental-health effects, and in some contexts may disproportionately benefit populations facing socioeconomic adversity.

This idea is sometimes described as the equigenic effect of green space:

Green environments may help reduce health inequalities rather than widen them.

However, two crucial nuances emerge:

1. Access is uneven. Disadvantaged communities often have less access to safe, high-quality green space.

2. Quality may matter more than proximity. A poorly maintained, unsafe park may not confer benefits—and may even produce stress.

This shifts the question from:

“Is there a park nearby?”

to: “Is the green space safe, biodiverse, welcoming, culturally relevant, and accessible?”

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Table 4. Observational evidence: long-term mental health and equity

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5.4. Causality and limitations

Observational research cannot fully eliminate confounding. For example:

• Healthier individuals may move to greener areas.

• Greener areas may also differ in income, healthcare access, or education.

• NDVI measures vegetation density, but not subjective experience of nature.

However, newer designs are improving causal inference:

• Longitudinal follow-up

• Natural experiments (e.g., greening vacant lots)

• Within-family or twin comparisons

• Fine-scale satellite and street-level imagery

The direction of evidence across multiple countries increasingly points in the same direction:Living in greener environments is associated with better mental health outcomes over time.

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5.5. Translational reflection for Shinrin-Yoku and urban policy

For practitioners and educators, this observational evidence offers an important reframing:

• Shinrin-Yoku sessions are powerful, but chronic exposure patterns shape population health.

• Advocacy for green infrastructure may be as important as individual guidance.

• Equity must be central—otherwise, green space can inadvertently drive displacement (“green gentrification”).

In practical terms:

• Support community-based green initiatives in underserved neighborhoods.

• Teach participants to see green space not only as therapy, but as civic right.

• Integrate research literacy into guide training so that practitioners understand both strengths and limits of current evidence.

6. Mechanisms: how might nature “get under the skin”?

When we ask whether nature “works,” the more scientifically precise question is:

Through which biological, psychological, and social pathways does nature exposure influence health?

It is rarely a single mechanism. Current models assume stacked, interacting pathways, where physiological regulation, cognition, emotion, behavior, and environment reinforce one another over time.　Below, we expand each pathway and then synthesize them into an integrative framework.

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6.1. Nervous system regulation

(Stress Reduction Theory – SRT)

Stress Reduction Theory proposes that natural environments facilitate a shift from sympathetic activation (“fight-or-flight”) toward parasympathetic dominance (“rest-and-digest”).

What happens biologically?

After brief exposure to forests or green environments, experimental studies frequently observe:

• ↓ Heart rate

• ↓ Blood pressure

• ↑ Heart rate variability (HRV) in some designs

• Reduced subjective stress

• Often ↓ cortisol (with methodological caveats)

These markers reflect changes in:

• Autonomic nervous system balance

• Hypothalamic–pituitary–adrenal (HPA) axis activity

• Vagal tone (linked to emotional regulation and resilience)

This suggests that natural environments may act as low-threat sensory fields that reduce vigilance demands and allow physiological settling.

Importantly, this is not merely relaxation—it is bioregulatory recalibration.

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6.2. Cognitive restoration

(Attention Restoration Theory – ART)

Modern environments place sustained demands on directed attention—traffic signals, screens, multitasking, social evaluation. This leads to attentional fatigue.

ART proposes that natural environments provide:

• Soft fascination (e.g., rustling leaves, flowing water, shifting light)

• A sense of “being away”

• Coherence without overload

This allows executive control systems to recover.

Observed outcomes:

• Improved attention task performance

• Reduced mental fatigue

• Greater perceived restorativeness

Cognitive restoration may indirectly improve emotional regulation, since attentional capacity is required to disengage from intrusive thoughts.

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6.3. Emotion regulation and rumination loops

Beyond general stress reduction, neuroimaging research suggests that nature exposure may alter activity in brain regions associated with maladaptive self-focused thought patterns (e.g., rumination).

Rumination is linked to:

• Depression risk

• Anxiety persistence

• Chronic stress activation

Nature walks have been associated with:

• ↓ Self-reported rumination

• Changes in subgenual prefrontal cortex activity (a depression-relevant region)

This suggests that nature may interrupt repetitive negative self-referential processing, allowing emotional experience to become more dynamic and less sticky.

From a mechanistic perspective, this connects:

• Physiological calming

• Cognitive widening

• Reduced self-focused looping

These layers likely reinforce one another.

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6.4. Behavioral pathways

Nature exposure does not operate only through direct physiological shifts.

Green infrastructure often enables:

• ↑ Physical activity (walking, cycling, play)

• ↑ Social contact and community cohesion

• ↓ Sedentary time

• Exposure to daylight (circadian regulation)

• Routine formation (habitual walking patterns)

Physical activity alone is associated with:

• Reduced depression risk

• Improved cardiometabolic health

• Enhanced neuroplasticity

Social connection buffers stress through oxytocin and other neurobiological pathways.

Thus, nature can influence health indirectly by changing behavioral opportunity structures.

This is why urban design matters: access shapes behavior, and behavior shapes biology.

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6.5. Environmental risk reduction

Natural environments can also act as protective buffers against environmental stressors:

• Urban trees reduce surface temperature (heat mitigation)

• Vegetation can attenuate noise (context-dependent)

• Green spaces may reduce exposure to certain pollutants

• Shade reduces UV stress (though pollen exposure may increase allergy risk in some contexts)

These effects are highly location-specific but can meaningfully influence:

• Cardiovascular stress

• Sleep quality

• Respiratory health

• Heat-related morbidity

In this pathway, nature reduces external stress load, which in turn reduces internal physiological burden.

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Table 5. Mechanistic pathways linking nature exposure to health

Mechanism	Primary System Affected	Immediate Effects	Downstream Health Impacts	Evidence Strength	Key Moderators
Nervous system regulation (SRT)	Autonomic nervous system; HPA axis	↓ HR, ↓ BP, ↑ parasympathetic activity	Reduced chronic stress load; cardiometabolic protection	Strong (experimental support)	Baseline stress; exposure duration; environment quality
Cognitive restoration (ART)	Executive attention networks	↑ Attention performance; ↓ mental fatigue	Better task performance; improved emotional regulation	Moderate–Strong	Cognitive demand prior to exposure; age; context
Emotion regulation & rumination	Prefrontal-limbic circuitry	↓ Rumination; altered depression-linked neural activity	Lower depression risk; improved affect stability	Emerging but compelling	Trait rumination; duration; solitude vs. social setting
Behavioral pathways	Physical activity systems; social networks	↑ Movement; ↑ social interaction; circadian exposure	Reduced depression; improved metabolic health	Strong (observational)	Accessibility; safety; amenities
Environmental buffering	Cardiovascular & respiratory systems	↓ Heat load; ↓ noise (context); pollutant mitigation	Lower cardiovascular risk; improved sleep	Context-dependent	Urban density; species composition; design

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6.6. An integrated model: stacked and interacting pathways

These mechanisms rarely operate in isolation.

For example:

1. A shaded park reduces heat load (environmental buffering).

2. Lower heat improves autonomic comfort (nervous system regulation).

3. Reduced vigilance allows attention to widen (cognitive restoration).

4. Reduced rumination enhances emotional flexibility.

5. Feeling safer and calmer increases likelihood of returning (behavioral reinforcement).

Over time, repetition of these cycles may contribute to:

• Lower allostatic load

• Greater emotional resilience

• Reduced psychiatric risk

• Improved cardiometabolic health

This is why even small acute shifts, when repeated, may scale into long-term outcomes observed in population studie