Gulf Coast study reveals surprising effects of sunlight on mental health

In a recently published study, Journal of Exposure Science and Environmental Epidemiology, A group of researchers assessed the association between exposure to solar radiation and depression and psychological distress in residents of the US Gulf Coast states.

Study: Association between solar radiation and mood disorders in Gulf Coast residents. Image credit: Mark Winfree/Shutterstock

background

Mental distress and depression are mood disorders characterized by emotional disturbances, where distress includes symptoms of stress, anxiety, and depression. From 1999 to 2018, the prevalence of distress increased from 16.1% to 22.6% and is associated with a range of health problems. Depression affects 8.8% of the population and has serious, long-term consequences on health and daily life. Climate factors such as solar radiation may influence mood disorders, but previous studies have produced mixed results, often with limited data resolution due to differences in methods, sample sizes, and geographic regions. Further studies with high-resolution exposure data are needed to clarify these findings.

About the Research

The current Gulf Longitudinal Follow-up Study (GuLF) is investigating health effects from the 2010 Deepwater Horizon oil spill. The study includes 32,608 adults aged 21 years and older who participated in the oil spill response or were trained for potential work. Participants were primarily from the Gulf region and were enrolled between March 2011 and March 2013. Home visits were completed with 11,119 participants, who underwent biological sample collection, functional measures, anthropometric measurements, and mental health screening. Written informed consent was obtained, and the study was approved by the National Institutes of Health (NIH) Institutional Review Board (IRB).

Depression and psychological distress were analyzed using the Patient Health Questionnaire-9 (PHQ-9) and the Kessler Brief Distress Inventory (K6). The PHQ-9 assesses depressive symptoms, with a score of 10 or higher defining depression, and the K6 assesses psychological distress, with a score of 13 or higher defining distress. The analysis included 10,217 PHQ-9 respondents and 8,765 K6 respondents.

Solar radiation exposure data was obtained from the Daymet database, which provides high-resolution (1 km x 1 km) climate data. Average solar radiation exposure was calculated over 7, 14, and 30 days prior to each home visit and categorized into quartiles. Temperature and humidity data were also included.

Associations between mental health status and solar radiation exposure were analyzed using generalized linear mixed models adjusting for demographics, lifestyle factors, temperature, and humidity. Sensitivity and stratified analyses were performed to assess effect modification. Statistical analysis was performed using R version 4.2.1, and significance was defined as p < 0.05.

research result

Of the 11,119 home visit participants enrolled in the study, approximately 78% were male, and the mean age was 43.8 years. The sample was primarily white (54.8%) and non-Hispanic (93.9%). Of these participants, 10,217 completed the PHQ-9 and 8,765 completed the K6. The correlation between the scales was .79. Of the 8,464 participants who completed both scales, 740 were depression only, 100 were distress only, and 529 were positive for both symptoms. Participants who were neither depressed nor distressed tended to be better educated, more likely to be employed, had higher incomes, and smoked less.

Spatial distribution of solar radiation (SRAD7, SRAD14, and SRAD30) at county/parish level indicates substantial exposure variation across the study area. Fully adjusted models controlling for temperature and relative humidity showed that higher SRAD7 levels were inversely associated with depression and distress, although this relationship was not monotonic. For SRAD7, prevalence ratios (PRs) of depression and distress decreased with higher exposure levels, but these associations were not statistically significant.

Seasonal variations were more pronounced for distress than for depression. In spring and summer, the PR for distress increased with increasing levels of SRAD7 and SRAD30, whereas in fall, both depression and distress showed non-significant decreases with increasing SRAD7. Specifically, in spring, the PR for SRAD7 ranged from 1.28 to 2.29 and in summer, it ranged from 1.09 to 1.67. In fall, the PR for depression ranged from 0.70 to 0.88 and the PR for distress ranged from 0.63 to 0.92.

Analyses stratified by age and sex showed that increasing levels of SRAD14 and SRAD30 were associated with a decreased PR of distress in participants under 50 years of age, but not in those over 50 years of age. Results were generally similar in men and women. Women, but not men, appeared to have a higher PR of depression with increasing SRAD14. Conversely, SRAD14 and SRAD30 were suggestively associated with a decreased PR of distress in men, but not women.

Furthermore, although greater exposure to solar radiation was generally associated with lower rates of depression and distress, patterns varied by season, age, and gender. The observed relationships suggest that the effects of solar radiation on mental health outcomes are complex and influenced by a variety of factors.

Conclusion

In summary, this study found that the relationship between mental health and solar radiation differed for different psychological outcomes. Psychological distress is more transient and therefore more sensitive to recent solar radiation levels, whereas depression, whose symptoms are more persistent, is less affected. Higher solar radiation levels reduced distress but were not associated with depression. Seasonal variations showed increased distress in spring and summer and decreased in autumn. Gender differences showed that men benefited more from higher solar radiation levels, possibly due to hormonal and outdoor activity factors.