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People who get more light during the day and less at night are less likely to suffer from mental health issues, including depression, anxiety, psychosis and self-harm, researchers have found in one of the largest studies of its kind.
Humans once lived by the sun’s rise and fall; our nights were lit only by the moon. But now we spend most of our time inside, our lives lit by the blue glow of screens.
Researcher Sean Cain has a new paper in Nature showing a strong link between daytime light exposure and good mental health. Credit: Simon Schluter
That artificial light may be messing with the setting of our circadian rhythm, some researchers believe, with consequences for our entire system, including our brains.
“As an input to the body, light is almost like injecting a drug,” said the study’s co-senior author, Monash University associate professor Sean Cain. “We know light is incredibly powerful on our physiology.”
The data has led Cain to reorient his life around light. During the day, he works from outside as much as possible; at night, his smart lights dim to a dull orange.
The new research, published on Nature.com on Tuesday morning, is pure scientific serendipity. Researchers in England were running a huge study trying to understand the relationship between genes and health, using wrist-based activity trackers on 103,720 people.
Those activity trackers also had light sensors, something lead author Angus Burns only discovered when digging through the schematics.
A special software script had to be created to extract the 14 million hours of light data, but at the end the researchers had by far the largest-ever database of light exposure and health outcomes.
The data revealed a strong linear association between more nighttime light exposure and self-reported major depressive disorder, anxiety disorder and four other mental health disorders.
Exposure to more daytime light was associated with a lower risk of all six disorders. Indeed, more daytime light exposure was associated with subjects saying they had an overall better quality of life.
“Modern humans spend about 90 per cent of the day indoors under electric lighting that is much too dim for what our bodies expect from the sun,” Burns, who is based at Harvard Medical School, said. “This has produced a humanity that is chronically underexposed to daylight and chronically overexposed to nighttime light.”
The study cannot definitively prove the role of light in mental illness, only that there is an association.
But there are several reasons to believe light is key. Exposure to light sets our circadian clock. If that clock is set wrong, we are at risk of a range of diseases.
People who work at night and sleep during the day face a much higher risk of mental and physical illness, for example.
And in a 2018 paper, Cain showed common antidepressants dramatically increase light sensitivity, suggesting part of their effect might simply be properly aligning the body clock.
Cain tries to work outside as much as possible to increase his exposure to daylight.Credit: Simon Schluter/The Age
“It’s an important story,” said Ian Hickie, co-director of health and policy at the University of Sydney’s Brain and Mind Centre. “And very likely to be causal.
“It impacts on the body clock, which when poorly entrained with the earth’s natural light-dark cycle is associated with a range of mental disorders, most importantly depression and other mood disorders.”
While the science is now being fleshed out, psychologists have long known light plays a key role in their patients’ mental health.
“We know disruptions to circadian rhythms cause disruption to mental health measures,” said Michael Berk, director of Deakin University’s Institute for Mental and Physical Health and Clinical Translation. “We know if you want to trigger an episode of bipolar, the very best way to do that is muck with the circadian clock.”
Bright light therapy has shown promise in treating depression and is widely used to treat seasonal affective disorder.
But Cain believes everyone can benefit from proper exposure to daylight. Work outside as much as possible and avoid light – particularly blue light – at night.
“It’s simple: get as much light as possible in the day, and as little as possible in the night, and keep it very regular,” he said.
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