The number of hours of sunlight needed every day to protect against dementia… as exposure may cut risk by 25%

An unsuspecting lifestyle factor may have a notable impact on the risk of developing dementia over a lifetime. 

A new study found that length of time exposed to daylight is associated with dementia risk.

Researchers from universities in China followed nearly 87,600 people for eight years. Participants were on average 62 years old and wore actigraphy devices on their wrists. 

These devices have built-in light sensors to measure light exposure and an accelerometer, which measures the acceleration of a person, or changes in motion, such as how quickly something is speeding up, slowing down or changing direction. 

Over the study period, 741 people developed dementia.

Based on the results, researchers concluded that people who remained indoors in dimly lit spaces had a higher risk of dementia at baseline but moving into brighter light conditions reduced that risk by 15 to 25 percent.

Results suggested that average daytime light exposure above 1,000 lux, which is the equivalent to a moderately bright light indoors or an overcast day outdoors, was associated with a 16 percent reduced dementia risk compared to people in dimmer light.

Spending 1.5 hours per day in light that is 3,000 or more lux – equivalent to outdoor light – was linked to an 18 percent lower risk of dementia. Between 40 and 45 minutes in very bright light, 7,000 lux or higher, was associated with a 17 percent decreased risk. 

Furthermore, less than 0.7 hours per day of bright daytime light was a stronger predictor of dementia than six established dementia risk factors, including obesity, alcohol consumption and traumatic brain injury. 

The protective factor of light was even more pronounced in people who were exposed to light at night, which can disrupt sleep. Interrupted and low-quality sleep has itself been linked to an increased risk of dementia.  

According to the study, published in the journal General Psychiatry, even if someone was exposed to more light at night, getting plenty of bright light during the day was still linked to a 30 to 38 percent lower risk of dementia.

People classified as ‘night owls,’ those who stay up later and sleep in late, saw a 40 percent reduced dementia risk when they got more daytime light. 

And even people with a genetic mutation that greatly increases the risk of Alzheimer’s disease saw a benefit to daytime light exposure – more daylight was linked to a 19 to 27 percent lower risk of dementia. 

Currently, seven million Americans are living with dementia, and that number is expected to nearly double by 2050. 

The researchers wrote that light exposure is a primary cue for the circadian system – the body’s internal 24-hour biological clock – which regulates cognition. 

However, association of light exposure with the risk of developing dementia remained unclear.

Now, based on their results, they added: ‘Further research should explore its role in dementia screening and inform the development of light-based interventions.

‘These findings position daytime light exposure as a novel, robust and measurable indicator of dementia risk. 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.’

Researchers posited that several mechanisms could be behind the protective association of light and dementia.

Disruptions in a person’s circadian system are a recognized risk factor for neurodegenerative conditions, including dementia, and researchers said that exposure to light may help fix problems with the system and boost sleep at night. 

They wrote: ‘Our [analyses]… offer preliminary support for the hypothesis that improvements in circadian rhythms may have contributed to the observed protective association.’

Additionally, the study stated that, historically, brain scans have shown that dementia is linked to a shrinkage in several key areas of the brain, while early research has suggested that getting enough light during the day may help slow down that shrinkage.

However, this research is still in the early stages and more work needs to be done to fully examine the relationship.