The Neurobiological Effects of Urbanizatio

Humans are extremely flexible and can live in a broad range of environments. But for the first time in human history, most of the world's population lives in urban areas. One hundred years ago, 20% of people lived in urban areas; by 2010, more than 50% of the global population lived in a city. By 2030, it is estimated that 70% of people will live in urban areas(1). In the United States and Canada, for example, 80% of the population lives in urban dwellings. As urban environments represent a deviation from the ancestral habitats of humans, the neurobiological impact of transitioning to city living should be examined. Considering the importance of humans maintaining close ties to the environment to acquire life-sustaining resources, there is little uncertainty that natural environments influenced the evolution of various neural functions ranging from learning to emotional responses.  Given that modern humans have retained their ancestral brains, it is interesting to consider the impact of transitioning to new habitats such as urban dwellings over the course of just a few generations.  The identification of health-related urbanization trends is essential to inform science-based policies for generating urban landscapes consistent with well-being and decreased susceptibility for psychiatric illness(2). Research indicates that urban living is associated with higher rates of psychiatric disorders, with rates as much as 30% higher for conditions such as mood disorders(3).   The characteristics of the urban environment that shape mental health, and the neurobiological mechanisms through which the urban environment may affect mental health are yet to be definitively determined.Environmental engagement, contingency building and emotional resilience Several aspects of both enriched and natural environments have been associated with positive effects on mental health. In contrast, vulnerability to psychiatric illness has been observed in unnatural urban settings. In addition to observations of increased rates of major depression diagnoses, increased rates of schizophrenia have been observed in urban populations, with evidence of a dose-response relationship between urbanicity and onset of this mental illness.  An enhanced state of stress associated with city living may be one factor that predisposes urban inhabitants for the expression of symptoms of psychiatric illness. When analyzed more closely, subjects currently living in urban settings exhibited increased amygdala activity (involved in fear processing) during a cognitively challenging task. Extending beyond the nervous system, numerous reports of positive correlations between natural environments and other health indices such as immune and cardiovascular health have been reported, systems that also contribute to adaptive neural functions(4).  The initial environmental studies conducted a half century ago have slowly changed the perception of the neural landscape once thought to be relatively inflexible throughout one's lifetime. The importance of the environment continues to be important for obtaining a thorough understanding of how the brain adapts to various changing environments to enhance survival. As modern human environments stray from our ancestral roots, a close examination of how these modifications impact neural responses and functions may explain causal factors for various mental illnesses, especially those that are expressed at increased rates in urban settings. Effects of Light Pollution on Neuroinflammation and Mood Since the introduction of electric lights around the turn of the 20th century, people have become increasingly exposed to relatively bright and unnatural light at night, both outside and inside their homes. This “light pollution” now affects 99% of the US and European population and ∼60% of the remaining world's population(5).  Electric lighting was a boon for developing industry and technology; It permitted people to work into the night, which drove economic development. Use of light at night continues to rapidly increase (6% increase per year) without careful consideration of its biological implications.Most people consider light at night a beneficial factor of modern life, which is why it was overlooked as a significant health risk factor until recently. However, because of the powerful effects that light has on the endogenous circadian system and upon downstream outputs such as hormone secretion, light at night exerts a significant effect on many physiological processes(6). It is now apparent that light at night has marked social, ecological, behavioral, and health consequences. Research indicates that depression rates are enhanced among city dwellers compared to people living in rural regions(6).   It is implicated that increased exposure to light at night among city dwellers disrupts brain circadian rhythms leading to impaired affective responses.  For instance, the Old Order Amish living in the US do not use electricity and