The sun does not just influence the nature of the space 300-600 million kilometres around it; it also affects several chemical processes at a microscopic level within our bodies.
From the production of Vitamin D to the regulation of calcium, the sun’s influence is now seen in certain chemical processes in the brain. This has led researchers to increasingly explore the effect of sunlight as a potential therapy to alleviate mood disorders such as depression.
How much light and when?
Experts already recommend that the ideal time to soak in ultraviolet (UV) rays of the sun is from dawn until around 9-10am. Doing so at other times might lead to an increased risk of developing complications such as skin cancers and light sensitivity.
“When it comes to sunlight, obviously, we have to look at the UV index of that particular area in a particular season,” says Dr Vikram Jada, consultant neuropsychiatrist at Dr Vikram’s Brain and Body Clinic in Bengaluru. “Getting exposed to sunlight when the UV index is high is not recommended because it will lead to other problems like skin malignancies or photosensitivity.”
The UV index is a measure of UV radiation and is at its highest during the four-hour period around noon. The index becomes lower as we move away from the equator but increases with elevation.
“Having good exposure to sunlight helps you to sleep better. In turn this will have a good effect on one’s mood and anxiety levels as well,” Dr Jada adds.
This happens because our bodies undergo physical, mental and behavioural changes in a 24-hour cycle called the circadian rhythms that are linked with the natural cycle of day and night, or light and dark. Research has shown that melatonin – a hormone that regulates this cycle – is produced in response to darkness and lets our bodies know when to sleep.
Any disruption of this cycle has been linked to loss of sleep and an increased risk of developing mental health conditions such as anxiety and depression.
Another proposed mechanism in which sunlight could influence mental health has to do with the dysfunction of the axis between the hypothalamus, pituitary, and adrenal (HPA) glands, which has been well documented as increasing risk of developing depression. Exposure to sunlight can produce beta-endorphins, an element of this HPA axis, which is linked with reducing stress and pain, which is also found to lower symptoms of depression.
This HPA axis also plays a significant role in regulating our sleep cycles, eating habits and cognition, researchers have shown. It is also why increasing blood beta-endorphins is one of the therapies to counter depression.
Weather, light and mood
The role of sunlight in regulating mood can be seen clearly in winters or in polar regions, in which certain individuals develop mood changes as their exposure to sunlight is reduced. Light therapy is already a recommended treatment for people with depression symptoms.
This is because sunlight affects not only the levels of melatonin in their body, but also those of serotonin, a happy hormone. Several researchers have found that higher exposure to sunlight leads to higher serotonin levels in the body, leading to a lesser incidence of depression.
“As practitioners, we recommend light therapy to individuals having depression due to weather change. This is called Seasonal Affective Disorder or SAD, and light helps alleviate the depressive symptoms,” says Dr Bharath Holla, assistant professor of psychiatry at the National Institute of Mental Health and Neurosciences (NIMHANS).
Light therapy uses artificial lighting sources that produce light of the same wavelength as that of sunlight at dawn.
Research has shown that the active form of Vitamin D, calcitriol, induces the production of serotonin and helps to elevate its levels in the brain by stopping its reabsorption by serotonin-secreting brain cells.
However, the jury of science is still out on how exactly this works: a serotonin imbalance can be caused by several reasons – a lack of receptors that bind with the available serotonin, a shortage of the amino acid tryptophan that is a precursor of serotonin and not necessarily due to lack of Vitamin D. This is also why Selective Serotonin Reuptake Inhibitors (SSRIs) are widely prescribed as medicine for treating depression.
D and depression
But evidence suggests that Vitamin D could play a much larger role in the incidence of mental health disorders. Medical practitioners say they still have a long way before they can conclude whether the link is coincidental; or the two are truly connected. But it is an idea that is gaining steam.
Researchers at St Mary’s Hospital in Seoul, South Korea, in 2012 conducted a meta-analysis of 11 case-controlled and five cohort studies. It showed that a 10 nanogram/millilitre increase of Vitamin D3 decreased the risk of depression in five of the 11 case-controlled and two of the five cohort studies.
The researchers concluded that there seemed to be an inverse relationship between Vitamin D3 levels and the risk of depression, adding that the results of their research warranted more studies to establish this association.
But the data used by researchers was based on a random-effects model, possibly excluding other factors that can cause depression among the individuals included in the studies.
“There are so many confounding factors,” says Dr Jada of Dr Vikram’s Clinic. “We need more studies, especially placebo-controlled and other treatment-related controls wherein we can actively compare the response with the D3 supplementation. Till then I don’t see it [deficiency of Vitamin D] as a direct causative factor of depression.”
But there is more evidence to suggest the link between Vitamin D deficiencies and risk of depression. Another study done by researchers at McMaster University in Ontario, Canada, in 2018 reviewed 13 studies involving over 31,000 individuals showing that those with low Vitamin D levels had a significantly higher risk of depression.
“There are some preliminary data from clinical trials that showed improvement in depression with Vitamin D supplementation,” Dr. Holla of NIMHANS says. “But on the other side, most of the studies are not of good quality and don’t really show a clear improvement in depression.”
Risks from supplements
Moreover, he warns that supplementing Vitamin D orally comes with a risk of overdose if not monitored and this can cause toxicity that affects the kidneys or heart. Exposure to sunlight on the other hand comes with no risk of overdose of Vitamin D and can be seen as an adjunct to therapy for depression.
But till researchers can pinpoint the exact mechanism in which a deficiency of Vitamin D causes depression, experts will continue to call for more studies and clinical practitioners will not prescribe Vitamin D supplements or regular exposure to sunlight as a treatment for depression, even though they suggest it is best to do so anyway.