Exposure to external light: understand how the habit can help with the risk of depression

If you already enjoyed life outdoors, know that a new benefit has been discovered: spending an average of 1.5 hours a day exposed to external light can reduce the risk of depression, regardless of genetic predisposition, according to a recent study published in “Translational Psychiatry”. The study found that both people who spent more time outside and those who spent less time had higher risks of developing depression.

Depression is a common mental condition that affects millions of people around the world, showing emotional symptoms, cognitive and physical, such as sadness, fatigue, difficulty concentrating, changes in appetite or sleep, and lack of motivation. In recent years, the number of cases of depression has increased globally.

Scholars around the world believe that a combination of genetic and environmental factors may be the main factors causing depression. Children of parents with a history of depression are three to four times more likely to develop the condition compared to children of non-depressed parents. Certain genes such as B3GALTL, FADS1, TCTEX1D1, XPNPEP3, ZMAT2, ZNF501 and ZNF502 have been associated with a higher risk of depression in previous studies.

the new ally

In addition to genetic factors, the availability of outside light has also been associated with the risk of depression. External light plays an important role in the body's physiological processes. When the skin is exposed to sunlight, a chemical reaction takes place that converts a skin-specific molecule into active vitamin D, which is essential for the human body. In addition, there is a recent discovery that links exposure to levels of the stress hormone (cortisol).

The study team, along with lead author Jing Lin, sought to examine the relationship between time spent outdoors and risk of depression, taking into account individual genetic predisposition. The researchers analyzed data from the UK Biobank Project, an extensive biomedical database containing genetic and health information on half a million participants in the UK.

Participants reported the amount of time they spent outdoors on a typical day during summer and winter. Genetic risk of depression was assessed based on polygenic risk scores that took into account genes previously associated with depression. Participants' medical records were used to assess the presence of depression. Data on smoking and drinking habits, physical activity, sleep duration, body mass index and other factors were also analyzed as they are considered relevant to the observation.

Now, a new study, which lasted about 12.5 years of analysis, found that 3.58% of participants were diagnosed with depression. These individuals had characteristics such as older age, higher frequency of women and smokers compared to the rest of the study sample. In addition, they rarely or never used sunscreen, had low education, high body mass index, slept less, were exposed to higher levels of air pollution, and had a greater genetic predisposition to depression.

Interestingly, it has been observed that people with depression tend to spend more time outdoors, on average, compared to individuals without the condition. However, further analysis revealed that the relationship between exposure to outside light and depression was not straightforward. The lowest risk of depression was found in individuals who spent an average of 1.5 hours a day outdoors, which is equivalent to one hour a day in winter and two hours a day in summer.

Those who spent more or less time than that had a higher risk of developing depression. Participants who spent less time outdoors had a 9% higher risk, while those spending more than 1.5 hours a day had a 13% higher risk. When combined with genetic factors, this risk difference increased to 34% to 35%, mostly in individuals with an intermediate level of genetic predisposition to depression.

The researchers suggest that the lack of external light can negatively affect the neuroendocrine system. Sunlight plays an important role in regulating the production of serotonin, a chemical that influences our mood. The scarcity of outside light can impair serotonin levels, leading to mood swings, especially during certain seasons.

However, it is important to question why spending more time in outside light would be associated with an increased risk of depression. According to the same researchers, this may occur because prolonged exposure to ultraviolet light reduces the production of melatonin, a hormone that regulates our sleep patterns, the decrease of which can interfere with our internal clock, contributing to psychiatric disorders such as depression. Prolonged exposure to UV rays can also stimulate the production of certain substances in our bodies, which can also encourage depression.