A study involving nearly 87,000 people from UK Biobank also found that exposure to bright daytime light reduced the incidence of mental disorders
“Once people understand that their light exposure patterns have a powerful influence on their mental health, they can take some simple steps to optimise their wellbeing. It’s about getting bright light in the day and darkness at night.” Sean Cain, assistant professor, Monash University
People who are exposed to artificial light at night time are more likely to develop psychiatric disorders such as anxiety, bipolar disorder, post-traumatic stress disorder (PTSD) and tendencies toward self-harm, a study has found.
The study, which involved nearly 87,000 people, was carried out by Monash University and the Turner Institute for Brain and Mental Health, both in Australia. It was published in the journal Nature Mental Health.
Among people exposed to high amounts of light at night, the risk of depression increased by 30%, while those who were exposed to high amounts of light during the day reduced their risk of depression by 20%. Similar results were seen for self-harm behaviour, psychosis, bipolar disorder, generalised anxiety disorder, and PTSD. These findings, the authors say, suggest that the practice of avoiding light at night and seeking brighter light during the day could be an effective, non-pharmacological means of reducing serious mental health issues.
“Taken together, our findings are consistent with bright daytime light and low night-time light strengthening circadian rhythms as an antecedent to more robust mental health. Patterns of bright daytime light and low night-time light serve to enhance the amplitude and stability of the circadian clock as well as align its timing appropriately with daily activities,” the authors write.
Sean Cain, an associate professor at Monash University and lead author on the study said that the findings would have “a potentially huge societal impact.” He added: “Once people understand that their light exposure patterns have a powerful influence on their mental health, they can take some simple steps to optimise their wellbeing. It’s about getting bright light in the day and darkness at night.”
The study’s 86,772 participants were all from the UK Biobank, and were examined for their exposure to light, sleep, physical activity and mental health. Cain said the impact of night light exposure was independent of demographic, physical activity, season and employment. “Our findings were consistent when accounting for shiftwork, sleep, urban versus rural living, and cardio-metabolic health,” he added.
“Healthy circadian rhythms are essential for mental health and wellbeing,” the authors write. “This biological system evolved under predictable conditions of bright light during the day and darkness at night to ensure stable, robust rhythms.”
The authors noted that the study had some limitations, however. While they found a strong correlation between patterns of light exposure and levels of mental illness, they could not prove causation. They add, however, that “the robustness of our findings to adjustment for confounders, including physical activity and sleep, provides support for our interpretation.”
The light monitoring, the authors write, was performed using a wrist-worn device, which is not designed to measure light at the ocular level, so the data only provide a “coarse estimate of the actual effects of light on the circadian system.” Because the actigraphy data and outcome variables were not measured simultaneously, with the variables measured more than a year later, they say it is feasible that the habitual light-exposure patterns of participants could have changed in the interim.
Cain says that modern life leads to the natural distribution of light and darkness being reversed: “Humans today challenge this biology, spending around 90% of the day indoors under electric lighting which is too dim during the day and too bright at night compared to natural light and dark cycles. It is confusing our bodies and making us unwell.”
Although the link between disturbed circadian rhythms and mental illness has been known for some time, the results of this study are nonetheless striking. Although there are some potential caveats about the data collection (eg the light was monitored via wrist-worn device, giving broad rather than precise measurements of light exposure), the size of the study, involving nearly 87,000 participants, suggests that the findings are reasonably robust. It is particularly noteworthy that, not only did increased exposure to artificial light at night time seem to increase the risk of a variety of mental disorders, but that increased exposure to natural light in the day time reduced the risk. This was the case even when allowing for confounding factors. We now need researchers to investigate whether it is possible to treat mental illness by restoring their patterns of light exposure to those required for healthy circadian rhythms.