It was found that the presence of available green spaces ^{[6][7][8][9][10][11]}[20,25,32,42,49,50] and parks ^[12][13][34,56] around the living environment had a positive impact on mental health. The presence of green space around the living environment tends to have a positive impact on mental health, and this may be due in part to the high likelihood of exposure to sunlight in green space and natural environments. In addition, as reported by Wang et al., air pollution such as PM2.5 may have a negative impact on mental health, partially because it reduces the duration of sunlight exposure. However, some of the studies includid ed in this study did not show positive effects on mental health in rural areas with a lot of nature and green spaces ^[14][6], suggesting that the use and purpose of green spaces and parks may be more important than their existence itself. In other words, although there are more extensive green areas in rural areas, they do not contribute much to mental health if their primary use is for farming as a work. In contrast, if the purpose of the green area is for exercise and recreation, it appears to have a positive effect on mental health, regardless of whether the area is located in an urban or suburban/rural area ^[8][32]. Other included studies ^[8][15][32,44] suggested the importance of exposure to the natural environment as well as opportunities for interpersonal interaction.

Furthermore, rural areas have a relatively large older population, which differs from the composition of the urban population, making it difficult to compare the effects of external and internal factors directly in terms of public mental health. Moreover, rural areas are often medically depopulated and poorly served by public transportation, which usually makes it difficult to access medical institutions specializing in mental health ^[15][44]. However, this reflects the reality of super-aging societies in certain developed countries, where further depopulation of rural areas may become a major problem in the mental health of the elderly. On the other hand, another study showed that the prevalence of depression was higher among the urban poor, which makes it difficult to interpret the relationship between the external environment and mental health based on regional classifications such as urban and rural areas ^[16][55]. Again, regarding the relationship between sunlight exposure and mental health, all of the included studies except one ^[17][7] showed that sunlight may positively affect people’s mental health ^{[14][18][19][20][21][22][23][24]}[6,8,12,21,27,37,43,58]. However, the study by Beute et al. did not show a direct effect of exposure to the natural environment and sunlight exposure on emotional states but profiled higher hedonic tone and energy levels and lower tension levels in environments with higher exposure to both natural environment and sunlight, suggesting that, at least in part, sunlight exposure contributes to improved human mental health ^[17][7].

3. Relationship between Internal Factors and Mental Health

Regarding the relationship between internal factors/lifestyle and mental health, a number of studies have reported that daily exercise habits ^{[25][26][27][28][29][30][31][32][33][34][35]}[14,17,18,23,26,33,38,46,51,52,59] have a positive impact on mental health in general, while sedentary lifestyle ^{[36][28][37][38][32][34]}[16,23,24,35,46,52] and long screen time ^[39][40][15,41] increase the risk of depression. Although several previous studies ^{[41][42][43][37][44]}[9,10,19,24,57] have shown that exercise does not always have a positive impact on mental health for clinical populations experiencing a variety of mental health problems, a study by Barton et al. reported that mood disturbances and low self-esteem generally improve after participation in any exercise program ^[42][10]. Buchan et al. ^[43][19] showed that increased moderate-to-vigorous physical activity was associated with decreased anxiety and depressive symptoms in men. In addition, a study by Hahn et al. ^[44][57] showed that outdoor work in winter, with its short daylight hours, did not have a particularly negative effect on the development of depression, but rather could have a beneficial effect on mood, suggesting that exercise itself may have, at least in part, an antidepressant effect. On the other hand, interestingly, a study by Julien et al. ^[37][24] found no significant association between the amount of walking and depressive symptoms when assessed cross-sectionally, but showed that subjects with more depressive symptoms tended to walk less frequently afterwards, indicating that longitudinally, depressive symptoms may cause a decrease in walking frequency.

4. Relationship between Mixture of Internal/External Factors and Mental Health

On the other hand, a mixture of internal factors such as physical activity as well as specific external environmental factors such as walking activity in rural areas and green spaces decreases stress and negative emotions ^{[45][46][47][35]}[40,45,47,59], increases memory performance ^[48][49][29,53], promotes emotional and cognitive recovery ^[49][53], and decreases activity in brain regions associated with mental illness ^[50][11]. Furthermore, a study by Shanahan et al. ^[11][50] showed that visiting a green space at least once a week for an average of 30 min or more can prevent depression by up to 7%, suggesting that daily habits such as frequent visits to green spaces may reduce the risk of developing depression. In addition, an exercise experiment ^[41][9] conducted under the condition of viewing images of a cycling course rather than an actual green space reported less mood disturbance and perceived exhaustion compared to the achromatic or red filtered image condition, indicating that green-related visual stimuli may also have a synergistic effect on the anti-stress effects of exercise. Other studies using light stimuli have also shown that exposure to green or blue light reduces anxiety ^[51][68] and increases calmness ^[52][69], suggesting a close relationship between light exposure and mental health. Therefore, moderate exercise in specific external environments or environments that mimic external environments may play an important role in the maintenance and improvement of mental health. Furthermore, with respect to external environmental factors, light stimulation at specific wavelengths may serve as a key to the effects on mental health. In addition, interestingly but paradoxically, living in a walkable neighborhood was associated with a slightly higher diagnosis of depression and level of antidepressant use, and walkability was associated with greater depressive symptoms in more deprived neighborhoods. While dense urban environments may provide opportunities for physical activity, they may also increase exposure to noise, air pollution, and social stressors, which may increase levels of depression ^[53][30].

5. Conclusions

The results showed that exposure to sunlight, exposure to and use of green spaces, and physical activity each positively affected mental health. The use of green space was also associated with exposure to sunlight and promotion of physical activity, suggesting that internal factors as represented by moderate physical activity under specific external conditions or in an environment that mimics the external environment may contribute to further improvement of mental health.