This article is part of the 2018 NMJ Oncology Special Issue. Download the full issue.
Min J, Min K. Outdoor light at night and the prevalence of depressive symptoms and suicidal behaviorsâ€Ż: a cross-sectional study in a nationally representative sample of Korean adults. J Affect Disord. 2018;227:199-205.
To assess the association between outdoor light at night (LAN) and prevalence of depression and suicide in a population of adults in South Korea
Design & Participants
This cross-sectional population study used data from the 2009 South Korean National Community Health Survey to compare severity and rates of self-reported depression (n=113,119) and suicidal ideation and/or attempts (n=152,159) with the intensity of ambient environmental outdoor LAN surrounding each participantâ€™s residential location.
Study Parameters Assessed
Intensity of outdoor LAN around each participantâ€™s residence was reported from the database of the Korean National Centers for Environmental Information, as captured by satellite imagery and assessed as radiance (nanowatt/cm2/steradian [sr]). This data was divided into quartiles (Q1-Q4), from the darkest rural areas (Q1: radiance <13.19) to the brightest urban (Q4: radiance >60.44).
Participantsâ€™ depression scores were measured using the Center for Epidemiologic Studies Depression Scale (CESD). Number of suicidal ideation events and past suicide attempts were provided by self-report.
Primary Outcome Measures
Prevalence of depression and suicide attempts/ideation in the various quartiles of LAN.
The association between depression/suicide and LAN was determined by statistical analysis, adjusting for participantsâ€™ age, sex, marital status, education, monthly income, smoking status, alcohol consumption, and physical activity level. Environmental data (nighttime noise level, airborne particulate matter [PM10], and residential proximity to parks or other green spaces) around each participantâ€™s residence was also included in model adjustments, as these factors have also shown direct influence on mood states.
There was a statistically significant relationship between outdoor LAN and measures of depression and suicidal ideation when using the fully adjusted model (controlling for multiple individual and neighborhood environmental factors). Compared to participants living in the darkest (Q1) rural areas, odds ratios (ORs) demonstrated a 22% to 29% increase in depressive symptoms (P<0.001) and 17% to 27% increase in suicidal ideation (P<0.001) for participants living in areas with greater outdoor LAN (Q2-4), in a dose-response relationship.
Outdoor LAN creates a condition often known as â€ślight pollution,â€ť which is increasingly recognized as a public health threat.1 Like many other forms of pollution it, develops insidiously, seeming to occur in the environmental background without causing noticeable acute harm. Only when examined en masse, in large-scale epidemiological studies like this current one, do the true detrimental effects of chronic exposure start to present themselves.
Like other forms of light exposure, outdoor LAN is thought to create its negative health effects through photon stimulation of receptors in the retina, activating nerves that innervate the suprachiasmatic nucleus (SCN), suppressing melatonin production and altering the function of serotonin and other neurotransmitters that determine mood.2 These LAN exposures are well-known for altering circadian rhythm and inflammatory metabolic pathways,3 and it has been shown that indoor LAN is linked with an increased rate of depression,4 thus supporting the â€śsleep hygieneâ€ť practice of maintaining a darkened bedroom.
This study is not able to show if darkening bedroom windows to block out outdoor LAN would increase serotonin or improve mood disorders. However, it is known that outdoor light pollution has multiple detrimental health effects to both humans and ecosystems, in a dose-dependent manner similar to the findings of this current study.5,6 This strongly suggests that environmental light pollution has physiological effects on all life, disrupting the natural circadian rhythms that arose from evolutionary adaptation to the 24-hour day-night cycle.
It is also interesting to wonder if the sense of loss of night sky viewing, rather than merely its absence, plays a role in contributing to mood disturbance.
Disruption of this cycle from outside LAN and subsequent effects on depressive symptoms and suicidal ideation are most pronounced in urban areas (Q4 in this study), where light pollution is greatest. This may be a contributing factor for the increased prevalence of depression and other mental illness in cities,7,8 an idea supported by multiple comparative brain scan studies.9,10 These experiences of â€śurban stressâ€ť are often attributed to ambient noise, air pollution, and lack of restorative green space, but this current study controlled for these factors, allowing for the possibility that high outdoor LAN may play a role. We already know that living in urban settings artificially shifts melatonin production cycles away from their natural circadian rhythms, and that spending time in more natural, low-LAN settings restores these patterns.11 It may be that urban light pollution is causing neurochemical and neurostructural changes in the brain.
However, the mood disorder effects of LAN may not be entirely physiological. Researchers in the field of ecopsychology talk about the â€śmissing sky factor,â€ť that is, how light pollution creates absence of nighttime sky and loss of the grandeur of infinite stars. This in turn can limit feelings of joy as well as experiences of awe and wonder that allow children (and adults) to ponder lifeâ€™s deeper meanings.12 The ability of natural scenes as a whole to produce awe and other positive psychological states is well known,13,14 and it may be that residents in areas of greater LAN are missing out on these experiences that allow viewing of the night sky and pondering of oneâ€™s own value and purpose in the cosmos. In an environment where it is rare to see more than a handful of stars against a â€ślight washedâ€ť sky, it may be easier to become depressed and suicidal without a higher source of stellar inspiration.
It is also interesting to wonder if the sense of loss of night sky viewing, rather than merely its absence, plays a role in contributing to mood disturbance. This association between depression and environmental loss has been measured in more terrestrial-based natural settings15,16 and has even been given the name of â€śsolastalgia,â€ť ď»żthe distress produced by environmental change.17 Loss of beloved natural features is a common occurrence in the modern worldÂ and could certainly be contributing to the increased prevalence of depression seen around the globe.18 In his excellent book End of Night, author Paul Bogard discusses the many ways light pollution is a factor in this trend.19
Fortunately, awareness of the negative health effects of light pollution and outdoor LAN, both physiological and psychological, is growing. An entire field of â€śmedical chronobiologyâ€ť is developing to address circadian rhythm imbalances resulting from too much light exposure, as this entire special issue of Natural Medicine Journal addresses. From an environmental perspective, theÂ International Dark-Sky Association (IDA) works to promote awareness of light pollution and its impact on human and ecological health. The IDA advocates for the reduction of outdoor LAN in both urban and rural communities, through the hooding of outdoor house lights, street lamps, and stadium lighting, as well as the transition away from high-intensity LED (light-emitting diode) lights that tend to produce more damaging blue light.20 No studies are available yet on the impact of this work on mental health rates, but outdoor LAN in certain areas has begun to decrease.21
A significant limitation of this study is the lack of individual outdoor LAN exposure measurement. Outdoor LAN data was taken from environmental datasets for given geographical areas and may not accurately reflect true outdoor LAN exposure for every individual. Similarly, no biomarkers were collected from participants in this study, leaving us to speculate about the mechanism of action for these findings. Further studies in this area may want to collect biomarker data (eg, melatonin) and provide each participant with body sensors to measure individual outdoor LAN exposure.
Because this was a cross-sectional study using self-report survey data, the validity of the findings are limited. However, it may not be ethical to conduct a more rigorous experimental study with human participants, given that the outcome measures are depression and suicidal ideation.
Based on the data findings of this and previous studies, it appears that outdoor LAN plays some role in the determination of mood states that influence mental health, likely through impact on circadian rhythm and neurotransmitter production, with possible contribution from psychological mechanisms. Reestablishing optimal environmental conditions that reflect naturally occurring exposures of nighttime darkness may be an effective way to decrease rates of depression and suicide while supporting greater individual and ecosystem health.