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.

“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.

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“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.

Support is available from Lifeline on 13 11 14 and Beyond Blue on 1300 22 4636.

Liam Mannix’s Examine newsletter explains and analyses science with a rigorous focus on the evidence. Sign up to get it each week.