“Outdoor light is very bright and can help strengthen circadian rhythms, which we think is a good thing,” says Erik S. Musiek, MD, PhD, a researcher, professor of neurology, and a codirector of the Center on Biological Rhythms and Sleep at Washington University School of Medicine in St. Louis. Dr. Musiek was not involved in the research.
Although the study shows a link between daytime light exposure and a lower risk of dementia, it doesn’t prove that daylight prevents dementia — it only suggests a link. But it adds to a growing body of research that suggests daily light exposure, sleep-wake patterns, and brain health are intertwined.
How the Research Was Done
The study included close to 88,000 adults in the UK Biobank who did not have dementia at the start. Participants were about 62 years old on average; about 57 percent were women.
Each person wore a wrist device for seven days while going about normal life. The device, called an accelerometer, had a built-in light sensor so it could measure both movement and light exposure during the day and night.
The study measured light in lux, a standard unit of brightness. For reference:
- Typical indoor lighting is between 300 and 500 lux.
- Average daytime light (moderately bright light level equivalent to an overcast day outdoors) is around 1,000 lux.
- Bright daytime light can be up to 5,000 lux.
Researchers then followed study participants for a median of 8 years. Over that time more than 700 people developed dementia.
The Link Between Daytime Light and Dementia Risk
The study data revealed that people who got more daytime light had a lower risk of developing dementia later.
Key findings included:
- People exposed to average daytime light above 1,000 lux had a 16 percent lower dementia risk than people with lower daytime light exposure.
- People who got at least 5,000 lux for about 40 minutes a day had a 17 percent lower dementia risk.
- People who got at least 3,000 lux for about 84 minutes a day had an 18 percent lower dementia risk.
The authors also noted that getting less than about 40 minutes of bright daytime light was a stronger dementia predictor than six established dementia risk factors in their model, including alcohol use, obesity, air pollution, and traumatic brain injury.
The link appeared strongest in people who may already be at higher dementia risk, including people carrying the gene variant APOE ε4, “night owls” (individuals whose natural body clock tends to run later), and people with higher nighttime light exposure.
Somewhat surprisingly, nighttime light exposure was not significantly linked with dementia risk in this study. “Nighttime light is typically a concern because it can suppress melatonin and disrupt circadian rhythms,” says Musiek.
Could People Who Go Outside More Just Be Healthier?
The design of the study leaves uncertainty about the exact relationship between sunlight exposure and dementia.
“It’s still unclear whether lower daytime light itself raises dementia risk, or whether people in the earliest stages of cognitive decline are already spending less time outdoors,” says Musiek.
People who get more daylight may also be healthier overall, more physically active, more socially engaged, or more likely to have routines that take them outside, he says.
Researchers tried to account for some of these factors by adjusting for many dementia risk factors and lifestyle habits, including physical activity, social isolation, smoking, alcohol use, diet, obesity, diabetes, high blood pressure, hearing loss, traumatic brain injury, and the season when the device was worn.
Daytime Light Helps Set a Person’s Circadian Rhythms — Does That Help Brain Health?
Light is one of the main signals that helps set the body’s circadian clock. In this study, the authors found clues that rest-activity rhythms and some brain-structure measures may partly explain the link between daytime light and dementia risk, though those findings were exploratory and need more study, they wrote.
“There are other studies linking physical activity, exercise, circadian rhythms, and sleep to decreased dementia risk, but this study provides direct information on light exposure,” says Musiek.
The theory is that light exposure improves circadian function and ultimately sleep — but the many reasons for why people get more sunlight make it difficult to tease that out, says Musiek.
Earlier research using heart monitor data linked less distinct day-night activity patterns — such as being less active during the day, more restless at night, or having peak activity later in the day — with higher dementia risk in older adults.
Strengths and Limitations of the Study
The study had several strengths, including its large size and its use of a wearable-device measurement rather than a reliance on subjects to remember how much light they got.
But the wrist device is not a perfect measure of light exposure. It can be covered by shirt sleeves, and it measures light at the wrist, not the light that reaches the eyes, which is what most directly affects circadian rhythms, says Musiek.
Other limitations are that each subject’s light exposure was measured for only seven days, so may not reflect their long-term habits. Plus, participants from the UK Biobank tend to be healthier and less socioeconomically deprived than the general population, which may limit how well the findings apply to everyone.
The follow-up time was relatively short for a dementia study, says Musiek. “Many neurodegenerative diseases may begin developing 10 to 20 years before diagnosis,” he says.
How to Use Sunlight to Support Your Circadian Rhythms
Getting as much outdoor light exposure as possible during the day is probably a good step to protect circadian rhythms and brain health, says Musiek. “This does not mean sunbathing, as we don’t want skin cancer,” he adds. (A good sunscreen or hat can help prevent that.)
Musiek’s recommendations for adding sun to your daily life:
- Get outside during the day when you can. Outdoor light is much brighter than indoor light and may strengthen circadian rhythms, even on cloudy days.
- Try a morning walk. It’s an easy way to get light exposure early in the day.
- Take lunch outside. Or you could sit by a large window if getting outdoors isn’t realistic.
Musiek also recommends keeping evening light dim. He notes that even though this study didn’t find a link between nighttime light and dementia risk, other research suggests avoiding bright light at night — such as a TV on in the bedroom or bright overhead LED lights after 9 p.m. — may help improve sleep.
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