Gulf Coast study uncovers the surprising effects of sunshine on mental health

By Vijay Kumar MalesuReviewed by Susha Cheriyedath, M.Sc.Jun 4 2024

In a recent study published in the Journal of Exposure Science & Environmental Epidemiology, a group of researchers evaluated the association of solar radiation exposure with depression and psychological distress among residents in the United States (U.S.) Gulf states.

Study: Association between solar radiation and mood disorders among Gulf Coast residents. Image Credit: Mark Winfrey / Shutterstock

Background

Psychological distress and depression are mood disorders characterized by emotional disturbances, with distress encompassing stress, anxiety, and depression symptoms. From 1999 to 2018, the prevalence of distress increased from 16.1% to 22.6%, linked to various health issues. Depression, affecting 8.8% of people, has severe, long-term impacts on health and daily life. While climate factors like solar radiation may influence mood disorders, previous studies show mixed results due to variations in methods, sample sizes, and geographic regions, often with limited data resolution. Further research using high-resolution exposure data is needed to clarify these findings.

About the study 

The present Gulf Long-term Follow-up (GuLF) Study investigates the health effects of the 2010 Deepwater Horizon oil spill. It includes 32,608 adults aged 21 and older who participated in oil spill response or trained for potential work. Participants, primarily from the Gulf region, were enrolled between March 2011 and March 2013. Home visits, completed by 11,119 participants, included biological sample collection, functional measures, anthropometrics, and mental health screenings. 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 Kessler Quick Inventory of Distress (K6). PHQ-9 assessed depression symptoms, with scores ≥10 indicating depression, while K6 evaluated psychological distress, with scores ≥13 indicating distress. The analysis included 10,217 PHQ-9 and 8,765 K6 respondents.

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

Associations between mental health status and solar radiation exposures were analyzed using generalized linear mixed models, adjusting for demographic, lifestyle factors, temperature, and humidity. Sensitivity and stratified analyses were performed to evaluate effect modifications. Statistical analyses used R version 4.2.1, with significance defined as p < 0.05. 

Study results

Among the 11,119 home-visit participants in the study, approximately 78% were male, with an average age of 43.8 years. The sample was predominantly White (54.8%) and non-Hispanic (93.9%). Of these participants, 10,217 completed the PHQ-9, and 8,765 completed the K6, with a correlation of 0.79 between the scales. Among 8,464 participants who responded to both scales, 740 were positive for depression alone, 100 for distress alone, and 529 for both conditions. Participants who were neither depressed nor distressed tended to have higher education levels, were more likely to be employed, had higher incomes, and smoked less.

The spatial distribution of solar radiation (SRAD7, SRAD14, and SRAD30) at the county/Parish level revealed considerable exposure variations across the study area. Fully adjusted models that controlled for temperature and relative humidity showed that higher levels of SRAD7 were inversely associated with depression and distress, although this relationship was not monotonic. For SRAD7, the prevalence ratios (PRs) for depression and distress decreased with higher exposure levels, though these associations were not statistically significant.

Seasonal variation was more apparent for distress than depression. During Spring and Summer, the PRs for distress increased with higher levels of SRAD7 and SRAD30, whereas in Fall, both depression and distress showed non-significant decreases with increasing SRAD7. Specifically, in Spring, PRs for SRAD7 ranged from 1.28 to 2.29, and in Summer, from 1.09 to 1.67. In Fall, PRs for depression ranged from 0.70 to 0.88, and for distress, from 0.63 to 0.92.

Analyses stratified by age and sex indicated that PRs for distress decreased with higher levels of SRAD14 and SRAD30 among participants younger than 50 but not among those aged 50 and older. Results were generally similar for males and females. Women appeared to have higher PRs of depression with increasing SRAD14, but not men. Conversely, SRAD14 and SRAD30 were suggestively associated with decreasing PRs of distress among males but not females.

Furthermore, while higher solar radiation exposures were generally associated with a lower prevalence of depression and distress, the patterns varied by season, age, and sex. The observed relationships suggest that the effects of solar radiation on mental health outcomes are complex and influenced by various factors. 

Conclusions 

To summarize, the study found that the relationship between mental health and solar radiation varied by psychological outcome. Psychological distress, being more temporary, showed sensitivity to recent solar radiation levels, while depression, with more persistent symptoms, was less affected. Higher levels of solar radiation were linked to decreased distress but not depression. Seasonal variations showed increased distress in Spring and Summer and decreased in Fall. Sex differences indicated that males benefited more from high solar radiation, possibly due to hormonal and outdoor activity factors.