The clocks have just gone back in much of Europe, and the afternoons are getting dark early. Good timing for a study about daylight.
What the researchers did
A team led by Hongliang Feng at Guangzhou Medical University in China looked at 87,577 adults from the UK Biobank who wore a wrist device. Besides counting movement, it measured how much light they were in, day and night. Then the researchers followed them for a median of 8.1 years. In that time, 741 people developed dementia.
What they found
People whose average daytime light was above 1,000 lux had a 16% lower risk of dementia. For scale, 1,000 lux is roughly an overcast day outdoors. A bright office is often well under that.
Spending longer in really bright light, 5,000 lux or more, was linked to an even lower risk.
The headline number: getting less than 0.7 hours, about 42 minutes, of bright daylight a day was a stronger predictor of dementia than six well-known risk factors.
Light at night, on the other hand, showed no clear link either way.
"Daytime light exposure may serve as a novel indicator of dementia risk," Feng said.
The honest catch
This is an association, not proof. People who are already a bit unwell, less mobile or more isolated may simply go outside less. The study can't show that more sunlight prevents dementia on its own.
Still, a daily walk in daylight is cheap, safe for most people and good for sleep and mood anyway. Lunch outside sounds like a fair bet.