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Why sunlight is essential for our health (and for preventing cancer).

Happy Summer! I’ve recently shared a few posts on Instagram about sunlight and cancer, and as we head into the sunniest stretch of the year, I wanted to share some more information here. ☀️

For the past 50 years, public health messages have warned us about sun exposure. We've been told to avoid the sun, wear protective clothing, and apply chemical sunscreens daily—even when staying indoors.

The idea is that UVB light damages DNA, triggering mutations that lead to cancer. But why do skin cancer rates continue to rise—despite decades of sun avoidance, widespread sunscreen use, and indoor lifestyles? There is always more at play.

Full-spectrum sunlight is actually essential for human health. Sunlight is energy. When it touches our cells, a cascade of effects take place, influencing our entire body. Humans evolved under the sun. We spent the majority of ancient history outside, with regular exposure to its light. Our bodies are designed to receive the sun’s inputs.

Not only that, but darkness, and lack of light exposure, is stressful on the body. Cortisol, the body’s primary "stress hormone” and prolactin rise during periods of darkness to help maintain blood sugar levels and suppress energy expenditure to preserve resources for survival.

Sleep acts as a buffer against the stress of darkness. When we sleep, cortisol levels drop, and the body can focus on repair and recovery. Interestingly, those who stay awake during the night experience higher cortisol levels compared to those who are asleep, suggesting that sleep is a necessary counterbalance to the physiological demands of darkness.

Today, many people are mimicking the stress of darkness by avoiding sunlight during the day and relying on artificial light that doesn’t replicate the benefits of full-spectrum sunlight. This light deficiency can chronically elevate stress hormones, disrupt circadian rhythms, and impair energy metabolism

Insufficient Sun Exposure Has Become a Real Public Health Problem: “Studies in the past decade indicate that insufficient sun exposure may be responsible for 340,000 deaths in the United States and 480,000 deaths in Europe per year, and an increased incidence of breast cancer, colorectal cancer, hypertension, cardiovascular disease, metabolic syndrome, multiple sclerosis, Alzheimer’s disease, autism, asthma, type 1 diabetes and myopia.” (PMID: 32668607).

The human body is complex and nuanced. The idea that sun exposure directly causes cancer is an oversimplification. Research suggests that what we are told to avoid might be exactly what we need the most, especially given the high rates of vitamin D deficiency these days.

Light is anticancer.

Infrared light - penetrates the skin and reaches deep into tissues, delivering warmth and stimulating mitochondria. This enhances ATP production, boosts cellular repair processes, and supports metabolic efficiency.

Warmth - cancer cells thrive in environments with reduced metabolic function and lower temperatures. Warmer conditions, often associated with higher latitudes, can inhibit cancer cell proliferation by supporting efficient cellular metabolism.

Vitamin D - activated by UVB light, is a potent cell differentiator. It regulates cell growth, reduces inflammation, and strengthens immune defenses. Higher vitamin D levels are linked to reduced risks of cancers like breast, colon, melanoma, and prostate.

Circadian Health - light regulates rhythms: sleep, metabolism, and hormone cycles. Disrupted rhythms from lack of sunlight or artificial light at night elevate cortisol and prolactin—stress hormones linked to cancer risk. Sunlight synchronizes the body, reducing stress and restoring balance.

A 2024 study analyzing U.S. cancer incidence data from 2016 to 2020 found that higher solar UVB exposure was significantly associated with lower incidence rates of several internal cancers, including:

• Bladder

• Brain

• Breast

• Endometrial

• Esophageal

• Gastric (stomach)

• Non-Hodgkin’s lymphoma

• Pancreatic

• Renal (kidney)

Sun Exposure Is Associated with Reduced Breast Cancer Risk among Women Living in the Caribbean: The Atabey Study in Puerto Rico “We found lower risk of breast cancer associated with greater sun exposure in a population living with high, continuous sun exposure. This beneficial finding should be placed in the context of other effects of sun exposure.” (PMID: 34810207)

A note on skin cancer.

It’s extremely important to avoid sunburns. While lack of sun exposure is linked to higher rates of internal cancers, it’s true that sunburns can cause skin cancer, especially basal cell carcinoma (BCC). This is different from melanoma. For more information on the relationship between melanoma and the sun, I made a post here.

BCC is a treatable form of skin cancer BCC arises in the epidermis (the lowest layer of the skin). It's strongly linked to chronic sun damage, particularly in fair-skinned individuals. However, it is rarely fatal. BCC grows slowly and very rarely spreads (metastasizes) to other parts of the body.

Avoiding burns is one of the most effective ways to minimize the risk of BCC.

Interestingly, the internal cancers that are associated with inadequate low sunlight exposure and vitamin D deficiency are more aggressive and have worse outcomes. For example, colon cancer has a 5-year survivability rate of 65%, while basal cell carcinomas have a 5-year survivability of 100%.

Each person has their own biological makeup, skin tone, stressors, and risk factors for skin cancer. So, each person should approach sun exposure with an understanding of their individual risk. The goal isn’t to fear the sun, but to work with it wisely.

Simply going outdoors and spending time in the sun doesn’t automatically mean someone will get cancer. It’s important to consider that these issues aren’t black and white, and there are countless factors that contribute to an individual’s risk of skin cancer. Some of these include:

• Hormones. Research shows a direct correlation between excess estrogen and melanoma. Elevated levels can create a microenvironment that allows melanoma to develop, proliferate, and metastasize. Melanoma cells typically express estrogen receptors. One study found a 53% increase in melanoma risk in women on HRT taking estradiol without progesterone (PMID: 35028367).

• Chemical exposure. Exposure to certain chemicals, such as those in pesticides, industrial pollutants, or even skincare products, can increase the risk of skin cancer. Benzenes and petroleum products are frequently linked to skin cancer and, incidentally, often used in sunscreens, creams, and lotions (PMID: 29636341).

• Diet: Like all cancers, diet plays a large role in skin cancer. A diet deficient in nutrients including vitamin A, C, D, and E, and minerals like zinc and selenium, can increase the risk, especially because they are important for mitigating cell damage (PMID: 25995818).

• Polyunsaturated fats (PUFAs). Ingesting or topically applying these fats can damage cells. They are highly prone to oxidation, especially when exposed to heat. High concentrations of PUFAs can accumulate in the hypodermis—the fat layer beneath the skin. When exposed to UVB light, these fats oxidize and can cause inflammation. This can induce DNA damage and increase the potential for cancer (PMID: 29636341)

• Inconsistent sun exposure. This is also another major risk factor. Getting large doses at one time without consistently exposing the skin can increase the likelihood of burns and cell damage.

Some tips for sun exposure.

I believe in smart sun exposure: protecting your skin from burns, taking breaks, and listening to what your body needs.

• Spend time outdoors as much as possible. Even in cloudy regions, the benefits of full-spectrum light (including red and infrared wavelengths) persist. Consistency matters—intermittent, intense sun exposure increases the risk of burning and DNA damage, while steady exposure helps the body adapt.

• Avoid sunburns. Sunburn is a clear sign of tissue damage and increases long-term skin cancer risk. Use non-toxic mineral sunscreens (like zinc oxide) when necessary, wear hats or protective clothing, and take breaks in the shade. Remember: the goal is sun exposure, not sun damage.

• Expose skin and eyes to natural light within an hour after waking up. Avoid sunglasses, contact lenses, or glass windows if possible—they filter key wavelengths. Just a few minutes outside can recalibrate your circadian rhythm and lower stress hormones.

• Build a ‘sun callus.’ Gradual sun exposure in the spring primes the skin for summer. This process boosts melanin production, strengthens antioxidant defense systems in the skin, and reduces the likelihood of burns later.

• Build tolerance slowly. Start with just 5–10 minutes of unprotected sun and increase gradually over days and weeks. The goal is to stimulate protective mechanisms—not overwhelm the body. Those with fair skin or long indoor histories may need even slower adaptation.

• Eat a nutrient dense diet. Nutrients like vitamin A (retinol), vitamin C, vitamin E, zinc, and saturated fats help protect against UV damage from the inside out. Antioxidants neutralize free radicals generated by UV light. A diet rich in meat, eggs, dairy, fruit, etc. supports skin repair, hormone balance, and proper melanin synthesis.

• Spend time outside during sunrises and sunsets. Early morning and late light contains more infrared and less UV. Infrared light stimulates mitochondrial function, improving blood flow, and reducing inflammation.

• Avoid excessive artificial blue light exposure. A 2016 study concluded, “artificial light at night is significantly correlated for all forms of cancer as well as lung, breast, colorectal, and prostate cancers individually.” Blue light after sunset disrupts your circadian rhythm, and increases cortisol—leading to poor sleep, impaired metabolism, and increased inflammation. Getting sun exposure, using candles, using incandescent light bulbs, sleeping close to sunset, and avoiding screens at night can all help to avoid excess exposure. 

When purchasing sunscreen, I look for:

• Micronized zinc oxide: This offers non-toxic SPF protection (needs to be at least 20%).

• Low in titanium dioxide: Titanium dioxide usually causes a white cast.

• Low in PUFAs: I avoid sunflower seed oil, almond oil, grapeseed oil, etc. These oils oxidize when exposed to heat and light and can cause cell damage.

• No benzenes: Many sunscreens contain benzenes which are known carcinogens.

• No fragrance: Fragrance has been linked to adverse reactions and usually contains undisclosed synthetic ingredients (up to 3,500 chemicals can be labeled as “fragrance”).

• Reef-safe (zinc oxide is safe),

Some good options:

• Santa Cruz Paleo Sun Balm

• Honeybee Hippie Herbal Tinted Mineral & Tallow Skin Protection (code: DRCONNEALY)

• Blue light blocking glasses from RA Optics (code: CONNEALY)

Achieving a balance between safe sun exposure and protection from UV rays is key to cancer prevention, maintaining health, and connection to nature.

It’s important to make peace with our environment and find a healthy relationship with the sun. ☀️

Talk soon ❤️,

Dr. C