Sunlight: Friend or Foe?

For many years, conversations about sunlight have tended to fall into one of two extremes. One message tells us that the sun is dangerous and should be avoided whenever possible. The other suggests that more sun exposure is always better because sunlight is “natural.” And as sometimes happens when there are two extremes, neither extreme reflects the full truth. Sunlight is a powerful environmental signal so it should be used - wisely! Our eyes, brain, skin, blood vessels, immune system, and endocrine system all respond to different wavelengths of light. These responses can help organize sleep, hormone rhythms, metabolism, circulation, vitamin D production, pigmentation, and immune activity. At the same time, excessive ultraviolet exposure can damage DNA, accelerate skin aging, suppress protective immune surveillance, and increase the risk of skin cancer. The goal is therefore not to fear the sun, but it is also not to bask in ultraviolet all day.

The different ways the body experiences sunlight.

Sunlight contains several forms of energy, each producing different biological effects:

●Visible light enters the eyes and provides powerful signals to the brain.
●Ultraviolet B, or UVB, acts more superficially in the skin and begins vitamin D production.
●Ultraviolet A, or UVA, penetrates more deeply and can release nitric oxide from stores within the skin.
●Both UVA and UVB can also produce oxidative stress, inflammation, collagen damage, DNA injury, and skin cancer when the exposure is excessive.

This is important because the benefits that are bountiful are found from sunlight exposure; however, this is not the same as prolonged, unprotected UV exposure. You can receive an important circadian and neurological benefit while walking outside fully clothed and wearing sunscreen (1, 2).

Your eyes set your body's rhythms.

One of sunlight’s most important effects begins not in the skin, but in the retina. Specialized cells in the eye detect environmental light and send that information to the suprachiasmatic nucleus, or SCN, a small region of the hypothalamus that serves as the body’s master clock.

The SCN uses light information to coordinate:

● melatonin production
● the daily cortisol rhythm
● sleep and wake timing
● alertness and cognitive performance
● body temperature
● appetite and digestive timing
● blood-pressure rhythms
● hormone release
● the timing of metabolic activity throughout the body

The SCN also helps synchronize smaller biological clocks in organs and tissues such as the liver, pancreas, skeletal muscle, fat tissue, and skin. In other words, daylight tells the body not simply that it is time to wake up, but that it is time for thousands of physiological processes to occur in the correct sequence. (2, 3)

Why morning and daytime light matter.

Bright outdoor light earlier in the day provides a strong signal that says:

It is daytime. Become alert now, organize metabolism for the active part of the day, and prepare for sleep later.

A large observational study involving adults from the UK Biobank found that spending more time in daytime outdoor light was associated with more favorable sleep, circadian, and mood-related outcomes. This does not prove that daylight alone caused every benefit, but it supports the importance of a bright day and a darker night. (4, 5, 6, 7)

More recently, a small randomized crossover study placed 13 people with type 2 diabetes in either natural office daylight or constant artificial office lighting for approximately 41⁄2 days. During the daylight condition, participants spent more time in a normal glucose range, used more fat for fuel, had changes in evening melatonin, and demonstrated shifts in skeletal-muscle clock timing.

This was an early, short-term study—not a replacement for nutrition, exercise, or medication. But it provides fascinating human evidence that light is not merely visual information. It can also be metabolic information. Importantly, the intervention involved working in daylight through office windows, not sunbathing. (8)

This gives us a practical and positive message:

Build a brighter day and a darker evening.

Spend time outside after waking when possible. Walk, drink your morning coffee outside, garden, exercise, or work near daylight. Keep your eyes open normally, but never stare directly at the sun.

Bare skin is not required for this eye-to-brain pathway.

Your skin is an endocrine organ.

The skin is much more than a protective covering. It contains nerve endings, immune cells, pigment-producing cells, hormone receptors, enzymes, and local signaling systems. It can manufacture, activate, and respond to a remarkable number of chemical messengers. One of its best-known endocrine functions is the production of vitamin D.

The vitamin D pathway begins with a photon

When UVB reaches the epidermis, it interacts with a cholesterol-related molecule called 7-dehydrocholesterol.

That begins a sequence:

1. UVB converts 7-dehydrocholesterol into previtamin D3.
2. Heat within the skin converts previtamin D3 into vitamin D3.
3. Vitamin D3 enters the circulation and is converted primarily by the liver into 25-hydroxyvitamin D.
4. The kidneys and other tissues can convert it into calcitriol, the hormonally active form.
5. Calcitriol binds to the vitamin D receptor, influencing gene activity in many tissues.

Human research has also demonstrated that skin cells called keratinocytes contain machinery that can produce active vitamin D locally. That means the skin is not simply the starting point for vitamin D production; it can participate in local vitamin D metabolism and signaling. (9,10)

Vitamin D supports calcium and phosphorus regulation, bone mineralization, normal muscle and nerve function, and immune activity. It functions less like a conventional vitamin and more like the precursor to a hormone-signaling system. (11, 12)

How vitamin D production varies.

The amount of vitamin D produced from sunlight varies according to:

● season and latitude
● time of day
● cloud cover and air pollution
● skin pigmentation
● age
● clothing
● sunscreen use
● the amount of skin exposed
● medications and health conditions

The skin also becomes less efficient at making vitamin D as we age.

For these reasons, there is no scientifically responsible rule that everyone should receive the same number of minutes of unprotected sun. A person with very fair skin in Florida at midday may receive a very different UV dose than someone with darker skin in New England during the winter.

Vitamin D is important, but a sunburn is never required to make it. When vitamin D is inadequate, food and carefully selected supplementation can address the problem without intentionally adding more UV damage.

Sunlight can release nitric oxide from the skin.

Another intriguing pathway involves nitric oxide. Nitric oxide is a signaling molecule that helps blood vessels relax. It supports vascular tone, circulation, and the delivery of oxygen and nutrients to tissues.

Human skin contains compounds such as nitrite and S-nitrosothiols that can serve as stored sources of nitric oxide. UVA can break down some of these compounds and release biologically active NO.

In a study of 24 healthy adults, UVA exposure increased blood flow and produced a temporary reduction in blood pressure through a mechanism associated with nitric oxide release from the skin.

An earlier human study similarly found that whole-body UVA exposure could lower systemic blood pressure through this pathway. (13 ,14)

This does not mean that UVA exposure should be prescribed as a treatment for high blood pressure. UVA also contributes to oxidative stress, collagen breakdown, photoaging, and skin-cancer risk.

It does, however, demonstrate something remarkable:

The skin contains chemical reservoirs that can influence the circulation throughout the body.

Sunlight is not only acting on the surface. It can initiate signals that extend beyond the skin.

Sunlight modulates the immune system.

It is common to hear that sunlight “boosts” the immune system, but immune modulation is a more accurate description.

The goal of a healthy immune system is not to be constantly more active. It needs to respond strongly when appropriate, calm down when the threat has passed, tolerate harmless substances, and avoid attacking the body’s own tissues.

UV exposure can change:

● cytokine signaling
● antigen presentation
● the activity of skin-resident immune cells
● regulatory T-cell function
● local inflammatory responses
● vitamin D-related immune signaling

Recent clinical and laboratory research found that narrowband UVB could promote regulatory T-cell responses that help reduce excessive immune activity. This is one reason carefully measured UV phototherapy can be effective for inflammatory skin disorders. Dermatologists use controlled narrowband UVB to reduce inflammation, slow excessive skin-cell growth, and calm an overactive immune response in conditions such as psoriasis. (15,16)

But this is another area in which dose matters.

The same immune-dampening effects that can be therapeutic when UV is precisely measured may become harmful with repeated excessive exposure. Chronic UV-related immune suppression can reduce normal surveillance against abnormal cells and contribute to the development and progression of skin cancer. (17)

Therefore, natural sunlight should not be used as an uncontrolled treatment for an autoimmune or inflammatory condition.

The lesson is not that immune modulation is bad. The lesson is that biology requires calibration.

Melanin is the skin’s adaptive protective response

Melanin is often discussed only in terms of skin color, but it is part of a highly organized protective system.

When skin cells experience UV-related stress, melanocytes increase melanin production.

Melanocytes then package pigment into structures called melanosomes and distribute them to surrounding skin cells. The pigment helps absorb and disperse some incoming ultraviolet energy, reducing how deeply it reaches vulnerable cellular structures. (18, 19)

This is an elegant adaptive response.

However, it is important to understand what a suntan means.

A tan demonstrates that the skin recognizes UV-related stress and increased pigment production in response. It provides some additional protection against later exposure, but that protection is limited.

A tan is not evidence that the preceding UV exposure was harmless, and it does not make additional unprotected exposure safe.

The goal of intelligent sun exposure is therefore not to produce a tan.

The goal is to receive the benefits of daylight while keeping cumulative UV injury as low as reasonably possible.

The skin has its own neuroendocrine stress-response system

The skin even contains elements resembling the body’s central hypothalamic-pituitary-adrenal stress system.

Skin cells can produce or respond to signaling molecules such as:

● corticotropin-releasing hormone
● POMC
● ACTH
● alpha-MSH
● beta-endorphin
● glucocorticoid-related signals

These pathways help the skin respond to environmental stress, inflammation, injury, and ultraviolet radiation. They also demonstrate that the skin communicates with the nervous, endocrine, and immune systems rather than operating as an isolated organ. (20)

The POMC pathway that contributes to melanin production can also produce beta-endorphin. Animal research suggests that repeated UV exposure can raise skin-derived beta-endorphin and reinforce UV-seeking behavior. This may be one reason some people experience tanning as particularly rewarding or find it difficult to stop. The strongest evidence for this dependency pathway comes from animal models, so it should not be interpreted as proof that UV tanning is a healthy way to improve mood.(21)

The safer way to obtain the neurological and mood benefits of light is through regular daytime outdoor exposure—not through intentional tanning.

Sunlight and health are a package deal.

When people go outside, sunlight is often accompanied by other health-promoting behaviors:

● walking and physical activity
● time in nature
● fresh air
● social interaction
● a break from screens
● improved circadian timing
● better sleep later that night
● a greater sense of connection and well-being

This is important because we do not need to isolate sunlight from the rest of a healthy life.

A morning walk may provide daylight to the eyes, movement to the muscles, a cardiovascular stimulus, exposure to nature, and stress relief—all at the same time.

The message should not be “avoid being outside.”

It should be:

Go outside often. Move your body. Enjoy the day. Manage your UV dose intelligently.

An intelligent sunlight plan

1. Build a bright day

Get outside regularly, particularly earlier in the day. A morning walk, breakfast outside, gardening, or time near natural daylight can help reinforce circadian timing.

The duration does not need to be perfect. Consistency matters more than turning daylight into another performance test.

2. Remember that daylight and UV exposure are not the same thing

Circadian and neurological benefits occur through light entering the eyes. You can obtain these benefits while wearing clothing, a hat, and sunscreen.

Do not stare directly at the sun.

3. Check the UV Index

The UV Index tells you how strong skin-damaging ultraviolet radiation is expected to be.

When the UV Index is 3 or higher, public-health guidance recommends using sun protection. The UV Index can change with season, location, altitude, cloud conditions, and time of day.(22)

4. Avoid sunburn completely

A sunburn is not a sign that vitamin D production or another health pathway has been optimized. It is evidence of excessive exposure and inflammatory injury.

Avoid deliberate tanning and tanning beds. Artificial tanning devices deliver carcinogenic UV without the natural circadian and behavioral benefits of being outdoors.

5. Use several forms of protection

When UV exposure will be significant or prolonged, combine:

● shade
● lightweight, tightly woven or UPF-rated clothing
● a wide-brimmed hat
● UV-protective sunglasses
● broad-spectrum, mineral based sunscreen with SPF 30 or higher (Alba Botanic, Elta MD)

Apply sunscreen to exposed skin and reapply approximately every two hours, as well as after swimming or heavy sweating. Sunscreen should be one part of a layered strategy—not permission to remain in intense sun indefinitely.

6. Individualize your plan

Extra caution is appropriate for people with:

● a personal or family history of melanoma or other skin cancer
● numerous or atypical moles
● fair skin that burns easily
● a history of blistering sunburns
● immune suppression or an organ transplant
● medications that increase photosensitivity
● extensive occupational or recreational sun exposure
● previous radiation treatment
● significant existing sun damage

People with darker skin have more natural pigment protection, but they can still develop skin cancer. In darker skin tones, melanomas may be more likely to appear on the palms, soles, or around and beneath the nails, making complete skin monitoring important for everyone.

7. Address vitamin D as a measurable health issue

When vitamin D deficiency is a concern, review diet, health history, medications, bone health, and laboratory findings with a clinician.

Do not use repeated tanning or burning as a vitamin D treatment.

8. Know your skin

Skin cancer is unusual among cancers because many warning signs can be seen. Regularly examine your skin from head to toe, including:

● the scalp
● behind the ears
● the back and shoulders
● between the fingers and toes
● the palms and soles
● beneath and around the nails
● areas normally covered by clothing

A partner can help examine the back and scalp. Photographs may also help you recognize meaningful change over time. Regular self-examination can help detect suspicious lesions when they are more treatable. (23)

Remember the melanoma ABCDEs:

● A — Asymmetry: one half looks different from the other
● B — Border: the edge is irregular, blurred, or poorly defined
● C — Color: the spot contains several colors or uneven pigmentation
● D — Diameter: larger lesions may be concerning, although melanomas can be small
● E — Evolving: the spot is changing in size, shape, color, elevation, or sensation

Also pay attention to the “ugly duckling”—a spot that looks noticeably different from the others.

A new or changing lesion, a sore that does not heal, or a spot that repeatedly itches, crusts, bleeds, or oozes should be evaluated rather than watched indefinitely.

The sun is not the enemy.

Sunlight helps organize human biology.

It provides the brain with time-of-day information. It helps coordinate sleep and hormone rhythms. It may influence glucose regulation and fuel use through circadian pathways. It begins vitamin D production in the skin. It can release nitric oxide and affect vascular signaling. It influences pigmentation, immune activity, and local neuroendocrine pathways.

Those are meaningful biological benefits.

But sunlight is powerful precisely because it can change cells and tissues. When the UV dose becomes excessive, those changes include oxidative stress, collagen breakdown, DNA mutations, immune suppression, and skin cancer.

The healthiest approach is not total avoidance—and it is not unrestricted exposure. It is intelligent balance: Bright days. Darker nights. Regular outdoor activity. No deliberate burning or tanning.

Thoughtful UV protection. Adequate vitamin D. And careful attention to the skin.

At Advance Wellness and Longevity, we want our patients outside—walking, exercising, gardening, traveling, and enjoying the natural world.

We also want them to protect the skin that must serve them for decades to come. Enjoy the sun as a biological signal, a source of daylight, and part of an active life.

Just remember that with sunlight, as with exercise, nutrition, medication, and most powerful influences on the body:

Timing, dose, and the individual matter.

Warmly,

Martin Katz, MD

Advance Wellness and Longevity

This newsletter is for educational purposes only and is not a substitute for individualized medical or dermatologic advice. Sun-protection and vitamin D recommendations should be personalized according to skin type, medical history, medications, geographic location, lifestyle, and prior skin-cancer risk.

REFERENCES:
1. https://www.nigms.nih.gov/education/fact-sheets/Pages/circadian-rhythms
2. https://www.who.int/news-room/fact-sheets/detail/ultraviolet-radiation
3. https://nigms.nih.gov/image-gallery/2569
4. https://pubmed.ncbi.nlm.nih.gov/34488088/
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11. https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/
12. https://ods.od.nih.gov/pdf/factsheets/VitaminD-Consumer.pdf
13. https://pubmed.ncbi.nlm.nih.gov/19797169/
14. https://www.jidonline.org/article/S0022-202X%2815%2936878-0/fulltext
15. https://pubmed.ncbi.nlm.nih.gov/40847567/
16. https://www.aad.org/public/diseases/psoriasis/treatment/medications/phototherapy
17. https://pubmed.ncbi.nlm.nih.gov/41339105/
18. https://www.jci.org/articles/view/119206
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21. https://pubmed.ncbi.nlm.nih.gov/24949966/
22. https://www.cdc.gov/skin-cancer/sun-safety/index.html
23. https://www.aad.org/public/diseases/skin-cancer/check-skin