Each morning, when we open our eyes to a bright new day, our brain's 'master clock' synchronizes our body and its rhythms with the Sun.
The more light that we are exposed to on a daily basis, the better it may be for our brains in the long run.
According to recent research among nearly 88,000 older adults, those who are exposed to more daily light are less likely to develop dementia over the next decade or so.
The longer the exposure to light, either natural or artificial, the stronger the association.
In fact, spending at least 42 minutes a day exposed to bright light (5,000 lux or more) was associated with a 17 percent lower relative risk of dementia.
That's a better predictor of dementia than six other traditional risk factors, including obesity, alcohol consumption, air pollution, and hearing loss.
For 'night owls', the association was even stronger. Greater daytime light exposure was linked with up to a 41 percent lower risk of dementia.
The findings, published in General Psychiatry, support the idea that light-based interventions hold promise for people at risk of cognitive decline.
"These findings position daytime light exposure as a novel, robust and measurable indicator of dementia risk," write the study authors, led by researchers at Guangzhou Medical University in China.
"They could inform future research into light-based interventions and public health guidelines that recommend increased daytime light exposure as a low-cost strategy to support brain health, particularly in high-risk populations."
Many dementia patients suffer from sleep disorders or disruption to their sleep-wake cycles, and yet surprisingly little research has investigated light exposure and its connections to cognitive decline.
The human body's circadian clock tells us when to wake up and go to sleep, and it is naturally synchronized with sunlight.
When light enters the eyes, it activates the brain's master clock, thereby regulating the rest of the body's daily rhythms.
Sunlight on the skin, meanwhile, stimulates vitamin D production, which is also tied to dementia risk.
Previous studies have shown that brighter nights and darker days can disrupt circadian rhythms, but it remains unclear how this affects brain health in the long run.
In the recent analysis, researchers examined data from the UK Biobank, in which participants wore a light sensor continuously for seven days.
Those who were exposed to more daytime light during this week-long period had a significantly lower risk of dementia roughly eight years later.
This was particularly evident for night owls (as opposed to morning people), individuals with higher nighttime light exposure, and those who carried genetic risk factors for dementia.
For instance, those who carry the APOE ε4 allele and are exposed to more daytime light had up to a 27 percent lower risk of dementia.
"No prior study has investigated associations between average daytime and nighttime light exposures and dementia risk," the authors note.
It's not known how light exposure might impact cognitive decline, but in this observational research, participants exposed to more daytime light tended to have greater brain volume or surface area in parts of the medial temporal lobe.
This region plays an important role in memory and is often one of the first areas affected by dementia.
It's also possible that light is impacting the brain in a more indirect way, too.
Previous studies, for instance, have found that daytime light levels can predict lower risks of major depressive disorder, which is a risk factor for dementia.
"Therefore, ensuring brighter daytime light exposure (e.g., ≥ 1000 lux) may be a simple means of preventing psychiatric disorders and dementia," the authors of this recent study suggest.
"This is particularly relevant given that people in modern society spend nearly 90 percent of their day indoors under artificial lighting, which typically provides insufficient brightness (∼300–500 lux) during the day."
Perhaps a brighter day today could lead to a brighter brain tomorrow.
The research has been published in General Psychiatry.
This article was fact-checked by Rebecca Dyer and edited by Rebecca Dyer. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.