Researchers have found a potential link between prolonged exposure to natural sunlight and negative effects on brain health. Their study, published in Scientific Reports, indicates that spending more than two hours in the sun each day could be linked to decreased brain volume and increases in brain structural markers associated with disease, particularly in individuals under 60 years old and men.
Sunlight is often celebrated for its benefits, such as boosting mood and facilitating vitamin D production, which is essential for maintaining healthy bones and immune function. Previous studies have suggested that the brain can be affected by sunlight too, with increased brain temperature leading to a reduction in blood flow as the body tries to cool the brain. This may increase the risk of brain damage.
The researchers were motivated to explore this topic due to the limited understanding of how sunlight exposure affects brain structure. To address this gap, Huihui Li and colleagues from Qingdao University in China analyzed data from the UK Biobank, a population-based, large-scale study.
In total, 27,474 participants were analyzed, with an average age of 55 years. These participants had undergone brain scans and reported their average daily natural sunlight exposure. The researchers categorized participants based on their reported sunlight exposure, distinguishing between those who received less than 1.5 hours, 1.5 to 3 hours, and more than 3 hours of sunlight each day. Using statistical methods, they compared differences in brain structural markers across these groups. Factors such as age, sex, physical activity, average sleep duration, skin color, and history of disease were also accounted for.
The findings revealed a concerning trend: participants who reported longer sunlight exposure tended to have smaller brain volumes and increased white matter hyperintensities, which are considered markers of brain aging and can be linked to cognitive decline. Specifically, those in the highest exposure group (more than 3 hours) showed significant reductions in total brain volume, white matter, and gray matter compared to those with shorter exposure times. This effect was particularly pronounced in men and individuals under the age of 60.
“The mechanisms by which prolonged sunlight induces damage to brain structure are not fully understood,” Li and colleagues explained. They proposed that, “[sun]light may…penetrate the skin and heat tissues, increasing brain temperature. Elevated brain temperature can alter [signal] transmission, leading to changes in brain function,” and also “the UV radiation in natural sunlight can damage immune cells in the body, triggering inflammatory responses that can lead to damage.”
Interestingly, the study also identified a nonlinear relationship between sunlight exposure and brain health. While exposure up to two hours was associated with positive outcomes, exceeding this duration led to negative associations with brain volume. The researchers noted that these effects were more significant during the summer months, indicating that seasonal variations may play a role in how sunlight impacts brain structure.
The authors hypothesized a mechanism for this finding: “most of the vitamin D in the body is synthesized from sunlight exposure to the skin, and moderate sunlight exposure can maintain adequate levels of vitamin D which participates in maintaining brain function.” Li and colleagues also suggested that, “exposure to sunlight can regulate the release of neurotransmitters such as dopamine and serotonin in the brain, contributing to brain health.”
But the study has limitations. As an observational study, it cannot definitively establish causation between sunlight exposure and changes in brain structure. Additionally, sunlight exposure time was self-reported by the participants, which may be subject to bias.
The study, “The impact of sunlight exposure on brain structural markers in the UK Biobank,” was authored by Huihui Li, Fusheng Cui, Tong Wang, Weijing Wang, and Dongfeng Zhang.