It is well known that people who receive plenty of sunlight during the day live longer, and the risk of winter depression (seasonal affective disorder) also rises in winter, when daylight hours are shorter. New research has revealed that people exposed to more intense light in daily life have a lower risk of developing dementia later.
Associations between wearable‐device‐measured daytime and nighttime light exposures and dementia risk: A prospective cohort study - Zheng - 2026 - General Psychiatry - Wiley Online Library
https://onlinelibrary.wiley.com/doi/10.1002/gps3.70039
Less Than 42 Minutes of Bright Light Daily Is a Stronger Predictor of Dementia Than Obesity, Study Finds : ScienceAlert
https://www.sciencealert.com/less-than-42-minutes-of-daily-bright-light-is-a-stronger-predictor-of-dementia-than-obesity-study-finds
Humans have a physiological activity cycle called the circadian rhythm (body clock), and exposure to daylight helps regulate circadian rhythms. Also, involvement in dementia risk is well known; vitamin D is produced in the skin when it is exposed to sunlight.
Many people with dementia suffer from sleep disorders or disturbances of their sleep/wake cycle, but little research has examined the relationship between light exposure and dementia risk. The research team from Guangzhou Medical University in China therefore analyzed data from the UK Biobank, a large biobank in the United Kingdom, and investigated the relationship between the amount of exposure to bright light and dementia risk.
The study included 87,577 participants in the UK Biobank. Their average age was 62.36 years, 56.98% were women, and none had developed dementia at the time they entered the study. The participants wore a wearable device on their wrist for seven days, which measured how much light they were exposed to, both during the day and at night.
Whether the participants developed dementia was determined from medical records, death registry data, and other sources. Over a median follow-up period of 8.1 years, a total of 741 participants developed dementia. The research team used these data to examine the relationship between the amount and intensity of light exposure during the one-week measurement period and the subsequent risk of developing dementia.
The results showed that the more time participants were exposed to bright light exceeding 1,000 lux, the lower their subsequent risk of developing dementia. For reference, a typical well-lit office has an illumination of approximately 400 lux, while a department store sales floor has approximately 500–700 lux, and 1,000 lux is said to be comparable to the lighting in a television broadcasting studio.Outdoors during the day, the illuminance is said to be around 10,000–30,000 lux, even when it is cloudy.
There is a trend toward lower dementia risk with greater exposure to bright light. People exposed to light above 5,000 lux for at least 42 minutes per day had a 17% lower risk of dementia. This association was stronger than the associations with known dementia risk factors such as obesity, alcohol consumption, air pollution, and hearing loss.
The association between bright light and dementia risk was particularly pronounced among night owls and people with genetic risk factors for dementia. For example, among night owls, greater exposure to bright light was associated with a 31–41% reduction in the risk of developing dementia, while among carriers of APOE ε4, a genetic risk factor for Alzheimer’s disease, the reduction was 19–27%.
The effect of light exposure on cognitive decline remains unclear, but participants with greater exposure to daytime light tended to have greater brain volume or surface area in some parts of the medial temporal lobe. The medial temporal lobe plays an important role in memory and is one of the first regions affected by dementia.
Daylight may also affect dementia risk in a more indirect way. For example, previous studies have found that the more daylight people receive during the day, the lower their risk of depression, which is a risk factor for dementia.
The research team stated, “Ensuring exposure to brighter daytime light (for example, 1,000 lux or more) may be a simple way to prevent mental illness and dementia. This is particularly important given that people in modern society spend about 90% of their day under indoor artificial lighting, which generally provides a level of brightness that is insufficient for daytime use (around 300–500 lux).”
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