The world’s largest study of light exposure and its impact on mental health, involving almost 87,000 people, has found that increased exposure to light at night increases a person’s risk of psychiatric disorders such as anxiety, bipolar disorder and PTSD severity, as well as self-harm. Importantly, the study also found that increasing light exposure during the day can act as a non-pharmacological means of reducing the risk of psychosis.
Those exposed to high levels of light at night increased their risk of depression by 30% – while those exposed to high levels of light during the day reduced their risk of depression by 20%. Similar patterns of results were seen for self-injurious behaviour, psychosis, bipolar disorder, generalised anxiety disorder and PTSD. These findings suggest that the simple 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 problems.
The study, led by Associate Professor Sean Cain from the Monash School of Psychological Sciences and the Turner Institute for Brain and Mental Health in Melbourne, Australia, is published today in the journal Nature Mental Health.
“Our findings will have a potentially huge societal impact,” Associate Professor Cain said. “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 all about getting bright light during the day and darkness at night”.
The study’s 86,772 participants, all from the UK Biobank, were assessed for their exposure to light, sleep, physical activity and mental health. Associate Professor Cain said the effects of night light exposure were also independent of demographics, physical activity, season and employment.
“And our findings were consistent when we took into account shift work, sleep, urban versus rural living and cardio-metabolic health,” he said.
Humans in the modern, industrialised age have literally turned our biological systems upside down. According to Associate Professor Cain, our brains evolved to work best with bright light during the day and then with almost no light at night.
“Humans today are challenging this biology by spending about 90% of the day indoors under electric lighting that is too dim during the day and too bright at night compared to natural light and dark cycles. This confuses our bodies and makes us unwell,” he said.
