Immunostimulatory Effects of Sunlight: More Than Just Vitamin D


While some health experts recommend lathering up with sunscreen whenever you go outside [1], recent publications note that sunlight may improve immune function. In addition to sunlight’s ability to increase vitamin D levels, scientists have shown that sunlight can improve the movement of infection-fighting immune cells and avert pathogen spread in hospital environments.

Scientists from the National Bureau of Economic Research published an article titled “Sunlight and Protection Against Influenza” [2]. The researchers noted that “sunlight strongly protects against getting influenza.”

“Our major result is that incremental sunlight in the late summer and early fall has the potential to reduce the incidence of influenza. […] [T]his study reinforces the long-held assertion that vitamin D protects against acute upper respiratory infections. One can secure vitamin D through supplements, or through a walk outdoors, particularly on a day when the sun shines brightly. When most walk, through herd protection, all benefit” [2]. The researchers also concluded that their findings are relevant to the current COVID-19 pandemic and possible future pandemics of respiratory diseases, including H1N1 [2].

Additional research has shown that an increased level of solar ultraviolet radiation (UVR) exposure is related to lower rates of cancers, autoimmune diseases, and allergies [3] [4]. Still, researchers are looking beyond the effects of vitamin D from sunlight and finding that sunlight improves T-cells' motility, a type of white blood cell that is an essential part of the immune system.

Scientists are using this data to understand better the overall effects of sunlight and the immune system and noting that infectious outbreaks, including influenza, appear to happen more often in winter months when a significant portion of people expose themselves to little or no UVR. Research has shown that UV light therapy can efficiently kill drug-resistant bacteria without causing skin damage [5].

Overall, researchers believe that they can explain sunlight’s benefits for the immune system by to main points:

First, sunlight’s well-established relationship with vitamin D. Skin cells receive and absorb UVB radiation from the sun, and following a chain of reactions, starting with the conversion of cholesterol in the skin, skin cells convert it into vitamin D. Vitamin D is then stored in the body’s fat cells. When needed, it is metabolized in the liver and kidneys and used throughout the body to regulate calcium, increase beta-endorphins [an endorphin produced in the skin that is a potent pain suppressor], and stimulate the immune system [6] [7]. Vitamin D deficiency is often associated with an increased risk of various diseases, including heart disease, cancer, depression, cognitive impairment, and bacterial and viral infections [8]. Since vitamin D is the only essential vitamin that the skin makes, sunlight exposure is vital for improving its levels. Still, if consistent sunlight exposure is not available, supplementation can be useful [8] [9].

Second, sunlight’s stimulation of infection-fighting T-cells. Research shows that T-cells are intrinsically photosensitive, and UV light (especially the blue light spectrum) initiates the production of H2O2 (hydrogen peroxide) in these cells, which strengthens their already-robust, immune-fighting capabilities [3] [10]. Doctors believe these effects help explain the benefits of UV therapy and are exploring treatment options for immunocompromised patients [3].

Beyond sunlight’s exclusive immune-stimulating benefits, scientists are investigating the possibility of incorporating sunlight into the health care environment. A study published in The Journal of Hospital Infection reviewed the roles of natural ventilation and daylight for controlling infection within healthcare environments noted that most airborne infectious microbes could not tolerate direct sunlight [11]. Researchers also found that sunlight may help regulate biological rhythms by acting as a time-cue, enhancing immune function [11].

Likewise, The Journal of Infectious Diseases discovered that sunlight inactivates the COVID-19 virus [12]. The researchers noted that simulated sunlight “rapidly inactivates the virus in aerosols” (think sneeze spray), with 50% and 90% of the viral particles inactivated within 6 and 20 minutes, respectively [12].

[Strange that health officials advocated to close beaches, right?]

Additional Immunomodulatory Actions

In addition to supporting healthy immune function, sunlight exposure is associated with a broad spectrum of positive health benefits, beginning with, of course, increased vitamin D levels. Vitamin D is not exclusive to the immune system. Research shows that an adequate D level helps improve one’s inflammatory response, decreases blood pressure, improving cognitive function, and reducing the risk for certain cancers [13] [14].

Skin Health. Studies have shown that regular sunlight exposure can have a positive effect on several skin disorders, including eczema, psoriasis, acne, pityriasis rosea, and vitiligo [15] [16] [17].

Supports Sleep. In a 2014 study, researchers investigated the effect of sunlight on office workers [18]. They noted that office workers who were exposed to more natural light experienced better sleep [18]. This effect is likely caused by sunlight’s ability to improve melatonin production, a hormone primarily released by the pineal gland that regulates the sleep-wake cycle [19]. Melatonin is also a vital hormone that plays a crucial role in inflammation and infection and can help suppress skin damage caused by UVR [19].

Mental Health. It is commonly known that many people may experience seasonal depression in winter months, a phenomenon dubbed seasonal depression or seasonal affective disorder. Clinically, this affects about 6% of the population [20]. In many studies, patients with seasonal affective disorder experienced improved moods and relief after sunlight therapy [21]. Accordingly, researchers have noted that increased vitamin D levels can increase one’s mood [22].

Cognitive Function. In a 2015 study, researchers hypothesized that sunlight is related to cognitive function through vitamin D metabolism [23]. Indeed, the researchers found a correlative function between sunlight and cognitive function, meaning increased sunlight is correlated with enhanced cognitive function. Additional studies have noted the long-term benefits of sunlight on cognitive function because sunlight helps support the natural sleep cycle, which leads to increased production of melatonin and serotonin [24] [25] [26].


The health benefits and potential for sunlight therapy are extensive. And, better yet, it is widely available, natural, and accessible for most to obtain. Still, researchers estimate that sunlight exposure accounts for 90% of most individual’s vitamin D levels [27] [28], so you are likely deficient if you are not getting much sunlight exposure.

If you are interested in purchasing a high-quality vitamin D product, check out Healthmasters’ Ultimate D3-10,000 with K2. If you have any questions about vitamin D supplements, please feel free to call our office at 800.726.1834.