The study, published in the journal Nature Mental Health, also independently found that greater light exposure during nighttime was associated with increased risks of major depressive disorder, generalized anxiety disorder, PTSD, psychosis, bipolar disorder and self-harm behavior. Authors note the impact of the circadian rhythm — or the 24-hour internal clock in our brain that regulates alertness and sleepiness by responding to light changes in our surroundings — as it pertains to many psychiatric disorders, namely the disturbance of this natural cycle.

“Therefore,” researchers said, “habitual light exposure may represent an environmental risk factor for susceptibility to psychiatric disorders.” Researchers looked to investigate whether exposure to natural light during the day and artificial light at night had any relation to psychiatric disorders involving circadian rhythm disturbances. They tested two primary hypotheses, that greater light exposure in the day is associated with lower risk for psychiatric disorders and better mood and that greater light exposure at night is associated with higher risk for psychiatric disorders and poorer mood.

“These hypotheses were motivated by the known effects of day and night-time light exposure on the human circadian system and the well-established links between circadian disruption and psychiatric disorders,” researchers wrote. Researchers examined data from 86,631 individuals in the UK Biobank database, with light exposure data gathered in 2013 when more than 100,000 UK Biobank participants took part in a seven-day physical activity and light exposure study. Participants wore an accelerometer with a light sensor on their dominant wrist for a week to record data on their movements and light levels.

Psychiatric data were later collected in 2016 as participants completed an online mental health questionnaire. Ultimately, the analysis proved both of the researchers’ hypotheses correct.  Results indicated that higher exposure to light at night was associated with a number of major depressive disorder, generalized anxiety disorder, PTSD and psychosis, while higher daytime light exposure was associated with lower odds of major depressive disorder, self-harm and psychosis. “Our findings demonstrate a consistent association of light-exposure patterns that are healthy for circadian rhythms with better psychiatric outcomes,” authors concluded.

They also noted that, in today’s modern world, humans tend to spend roughly 90% of the day indoors, with our light-exposure patterns being less bright in the day and more bright at night than at previous points in our evolutionary history.  “Addressing this deviation from our natural light/dark cycles may improve the general mental health of people in industrialized societies,” they said. The study provides new insight on the topic as the largest examination of objectively measured light exposure and mental health to date.

Still, it’s not without its limitations. While there are “well-supported causal mechanisms” linking bright nighttime light and dim daytime light with circadian disruption, and circadian disruption to mental health, authors acknowledged the possibility of reverse causation, highlighting the need for future longitudinal studies. “However, the robustness of our findings to adjustment for confounders, including physical activity and sleep, provides support for our interpretation,” they state.

Authors also note that light monitoring was performed using a wrist-worn device which was not resigned to measure light at the ocular level. Additionally, the light monitoring and outcome variables were measured with almost two years between, so it’s possible that light exposure patterns changed during that time.  Still, as we continue broadening our horizons and exploring new and innovative solutions for mental health, the findings could prove useful for future approaches. “These results suggest that light-exposure interventions may act in a transdiagnostic manner to improve mental health by strengthening circadian rhythms,” researchers write.

“Brighter days and darker nights may be a simple, freely available, non-pharmacological intervention to enhance mental health that is easily implementable in a community setting.” Read less