Sunlight is more than Vitamin D: what sensible sun exposure really does for your health, based on insights by Dr. Michael Holick and analysis by Dr. Joseph Mercola

Who are Dr. Michael Holick and Dr. Joseph Mercola? 

Dr. Michael F. Holick is a physician, biochemist, and professor at the Boston University School of Medicine. Since the 1970s, he has studied vitamin D synthesis and calcium metabolism, uncovering the active form of vitamin D (1,25-dihydroxyvitamin D) and how sunlight initiates its production in the skin.

Dr. Joseph Mercola is a physician trained in natural medicine. In his March 2025 article, he summarises Holick’s findings and highlights their practical implications for public health and daily well-being.

Vitamin D: not just a supplement, but a natural product of light

One of Dr. Holick’s core findings is that vitamin D produced in the skin acts differently from that consumed in supplement form:

• Longevity and bioavailability: Vitamin D made in the skin stays in the bloodstream two to three times longer than oral vitamin D. Nearly 100% of it binds to transport proteins, making it highly effective and stable.
• Self-regulation: Unlike supplements, which can cause overdosing, the body naturally regulates how much vitamin D it produces from sunlight, preventing toxicity. Excess production is neutralised by UV light, converting it into inert by-products.

Sunlight triggers the production of additional health-boosting compounds

Dr. Holick has shown that sun exposure initiates a cascade of beneficial biological effects beyond vitamin D:

• Nitric oxide (NO) – A molecule that dilates blood vessels, lowers blood pressure, and improves circulation. It is released almost immediately during sun exposure, enhancing cardiovascular function.
• Beta-endorphins – These natural pain relievers and mood enhancers are produced in the skin under UV exposure and contribute to the feeling of relaxation many people experience after being in the sun.
• DNA repair enzymes – UVB exposure activates the body’s repair mechanisms, which help counteract initial damage. This suggests that the body is well-equipped to manage sunlight’s impact when exposure is moderate and controlled.

Sun exposure, longevity, and immune health

Several studies referenced by Dr. Mercola and Dr. Holick indicate that avoiding sun exposure is associated with reduced life expectancy. One large Swedish study even found that sun avoiders had similar mortality risks to smokers.

Sunlight also plays a significant role in supporting immune function:

• It helps reduce the risk of multiple sclerosis, type 1 diabetes, and rheumatoid arthritis.
• Vitamin D regulates inflammation, enhances the body’s ability to respond to infections, and modulates immune responses.

Clinical trials led by Dr. Holick demonstrated that vitamin D supplementation influences the expression of hundreds of genes involved in cell apoptosis, immune defence, and DNA repair.

In one study, vitamin D-deficient adults were given 600 IU, 4,000 IU, or 10,000 IU daily for several months. Blood samples were analysed for gene expression changes in white blood cells:

• 600 IU/day affected over 150 genes
• 4,000 IU/day affected around 300 genes
• 10,000 IU/day affected more than 1,200 genes

It’s worth knowing that the human genome contains approximately 20,000 to 22,000 protein-coding genes (i.e., functional genes), most of which are active only in specific cells, at specific times, or under certain biological conditions. Is an effect on 150 to 1,200 genes significant? In the context of molecular biology and immunology, changes in the expression of several hundred genes in response to a single substance (in this case, vitamin D) represent a very strong effect. For comparison, many drugs influence only a dozen to a few dozen genes, while chemotherapy often alters the expression of 200–400 genes in cancer cells. The effect of vitamin D on over 1,200 genes (at a dose of 10,000 IU/day) is comparable to the action of active biological drugs.

Sunlight regulates circadian rhythms and sleep quality

Daily exposure to natural sunlight—especially in the morning—helps synchronise the body’s internal clock (circadian rhythm). This affects the production of melatonin, the hormone responsible for regulating sleep.

People who get regular sunlight exposure are more likely to:

• Fall asleep more easily
• Enjoy higher-quality, restorative sleep
• Maintain stable energy and mood throughout the day

How to practise safe and effective sun exposure

Dr. Holick promotes a concept called “sensible sun exposure.” The goal is to receive the benefits of sunlight while avoiding overexposure. His key recommendations:

• Morning light – Spend 10–15 minutes outside shortly after waking (without sunglasses) to reset your circadian rhythm.
• Midday sun – Expose 30–40% of your body (e.g., arms, legs, back) for 10–30 minutes, depending on skin type. Never stay in the sun long enough to burn.
• Strategic use of SPF – High-SPF sunscreens block up to 97.5% of UVB rays, which are crucial for vitamin D synthesis. Apply SPF only to high-risk areas like the face, hands, and ears—not the entire body.
• Be mindful of diet – High consumption of seed oils (rich in linoleic acid) can increase the skin’s susceptibility to oxidative stress from UV. Reducing such oils can improve sun tolerance.

Conclusion

Sunlight is not just a source of warmth or a means to tan—it is a biological signal that activates and regulates many systems in the body. It plays a role in cardiovascular health, immune regulation, DNA repair, mood, and sleep. While sunburn and overexposure are harmful, total sun avoidance can be equally damaging.

Sensible sun exposure reflects the way humans have lived for thousands of years—outdoors, in natural light, in tune with nature. As Dr. Holick puts it: “We were designed by nature to be exposed to sunlight daily—not just for vitamin D, but for a whole range of processes that support our health and vitality.”

If you’d like to learn more, watch the interview with Dr. Holick – “How Sensible Sun Exposure Benefits Your Health – Interview with Dr. Michael Holick.”