A new study finds that exposure to light at night disrupts our internal clocks and increases the risk of mental illness, while exposure to light during the day reduces this risk. This discovery provides a simple and effective non-pharmacological means to improve mental health.
Our internal clock, or circadian rhythm, regulates cycles of alertness and sleepiness by responding to changes in light in the environment. While other cues such as exercise, social interaction and temperature also influence circadian rhythms, light remains the most powerful influence.
It is well known that circadian rhythm disruption is a common feature of many psychiatric disorders. It makes sense, then, that exposure to light is a modifiable environmental risk factor for mental illness. To conduct the study, Monash University researchers led one of the world's largest studies examining the impact of daytime and nighttime light exposure on the risk of mental illness.
Researchers recruited 86,772 adult participants from the UK Biobank and examined their light exposure, sleep, physical activity and mental health. They found that people exposed to bright light at night had a 30% increased risk of depression, while those exposed to bright light during the day had a 20% lower risk of depression.
Similar patterns were found for self-injurious behavior, psychosis, bipolar disorder, generalized anxiety disorder, and post-traumatic stress disorder. The findings were consistent when accounting for demographic factors, physical activity, sleep, shift work, living environment and cardiometabolic health.
"Our findings will have potentially huge social implications," said Sean Cain, one of the study's co-authors. "Once people understand that their light patterns can have a powerful impact on their mental health, they can take some simple steps to optimize their health. This is having bright light during the day and darkness at night."
Researchers say modern life - especially artificial light and the light emitted by devices such as mobile phones, computers and TV screens - confuses our internal clocks and challenges the way our brains evolved to work best in bright light during the day.
"Humans today challenge this biological law by spending approximately 90 percent of the day indoors under electric lights that are too dim during the day and too bright at night compared to the natural light-dark cycle," Kane said. "It confuses our bodies and makes us feel sick."
The results of this study suggest that avoiding light at night and seeking it during the day may be a simple and effective non-pharmacological way to improve mental health.
The study was published in the journal Nature Mental Health.