A new study has found that people who spent an average of 1.5 hours per day exposed to outside light had the lowest risk of depression regardless of their genetic predisposition for depression. People who spent more or less time outside had higher risks of depression. The study was published in Translational Psychiatry.
Depression is a common mental health condition that affects millions of people worldwide. It involves a combination of emotional, cognitive, and physical symptoms, such as feelings of sadness, fatigue, difficulty concentrating, changes in appetite or sleep patterns, and a lack of motivation. The number of patients with depression has been on the rise in recent years worldwide.
Depression is believed to be caused by a combination of genetic and environmental factors. Children of parents with a history of depression are three to four times more likely to develop depression themselves compared to children of non-depressed parents. Studies have identified certain genes, such as B3GALTL, FADS1, TCTEX1D1, XPNPEP3, ZMAT2, ZNF501, and ZNF502, that are associated with an increased risk of depression.
The risk of depression has also been associated with factors such as the availability of outdoor light. Outdoor light is known to affect important physiological processes in the body. When the skin is exposed to sunlight, it triggers a chemical reaction that converts a specific molecule in the skin into active vitamin D that the human body needs. Exposure to outdoor light has also been found to decrease concentrations of the stress hormone cortisol.
Study author Jing Lin and her colleagues wanted to examine the association between the time spent in the outdoor light and depression risk. They were interested in determining the time spent in the outdoor that would be optimal to compensate for individual genetic predisposition for depression. They analyzed data from the UK Biobank project.
The UK Biobank is a large biomedical database containing detailed genetic and health information from half a million participants from the United Kingdom. It was started between 2006 and 2010 with participants from England, Scotland and Wales and has been augmented with additional data since. For this study, researchers analyzed data of 380,976 White British participants that were aged 38-73 years at the start of the study.
Participants reported the duration they spent outdoors on a typical day in summer and in winter in a questionnaire. Genetic risk of depression was assessed through a polygenic risk score that took into account genes that were identified as being linked to the risk of depression in previous studies. Assessment of depression was based on participants’ medical records. Aside from these, the researchers also analyzed data on patients smoking and drinking habits, physical activity, sleep duration, body mass index, various other diseases, taking of vitamin D supplements, use of sunscreen, and a number of other factors.
Results showed that 3.58% of participants were diagnosed with depression during the 12.5 years analyzed in the study. Participants who developed depression tended to be older, more often female, and more often smokers compared to the rest of the study sample. They also never or rarely used sun protection, were more likely to have low education, high body mass index, to sleep less, be exposed to higher levels of air pollution and have a higher genetic risk of depression.
Individuals with depression tended to spend more time on average in outdoor light compared to individuals without depression. However, further analysis revealed that the relationship between outdoor light exposure and depression was not straightforward. The lowest risk of depression was found in individuals who spent 1.5 hours per day outside on average, which equated to 1 hour per day in winter and 2 hours per day in summer.
Participants spending more or less than that had a higher risk of depression. Those spending less time than that had a 9% higher risk, while those spending more than 1.5 hours per day had a 13% higher risk of depression. The difference in risk increased to 34%-35% when it was paired with genetic factors. This combined effect of genetic risk and outdoor light exposure was particularly pronounced in individuals with an intermediate level of genetic risk for depression.
The researchers said a lack of outdoor light might negatively impact the neuroendocrine system. Serotonin is a chemical that influences our mood, and sunlight helps regulate its production. When we don’t get enough outdoor light, it can disrupt serotonin levels, leading to changes in mood, especially during certain seasons. Lack of outdoor light exposure can also reduce the production of vitamin D, which has been associated with depression and its seasonal patterns.
But why would spending more time in outdoor light be linked to an increased risk of depression? This might be because prolonged exposure to UV light reduces the production of melatonin, a hormone that helps regulate our sleep patterns, the researchers said. Reduced melatonin can disrupt our body’s internal clock, which can contribute to psychiatric disorders like depression. Furthermore, long-term UV exposure can stimulate the production of certain substances in our bodies, like IL-1β and IL-6, which are linked to inflammation and can play a role in depression.
The study gives an important contribution to the scientific understanding of risk factors of depression. However, it should be noted that the outdoor light exposure assessments were solely based on self-report data. Additionally, all study participants were white British people. Studies using more objective measures of outdoor light exposure and conducted on different populations might not yield equal results.
The study, “Association of time spent in outdoor light and genetic risk with the incidence of depression”, was authored by Jing Lin, Hongxi Yang, Yuan Zhang, Zhi Cao, Dun Li, Li Sun, Xinyu Zhang, and Yaogang Wang.
