Sun's Out - Guns Out!
The Essential Role of Consistent Sun Exposure in Human Metabolic Health!
Modern lifestyles have dramatically altered the human relationship with sunlight. Most people today spend over 90% of their time indoors, primarily under artificial lighting (Klepeis et al., 2001). This represents a profound shift from our evolutionary context, in which daily exposure to natural sunlight shaped critical aspects of our physiology over eons. Compounding the problem is the modern tendency to go from having almost no direct sun exposure for most of the year, to then getting “as much as possible” during summer holidays or tropical vacations – particularly true of those of us living in temperate climates – which results in sunburn and long-term skin damage... This erratic modern exposure pattern then contributes to and reinforces the growing cultural narrative that sunlight is inherently dangerous and best avoided altogether.
However, this narrative is misleading because while unprotected overexposure to ultraviolet (UV) radiation carries clear risks, emerging scientific literature underscores that regular, controlled exposure to natural sunlight is vital for optimal metabolic health. From circadian regulation and vitamin D synthesis to immune function, nitric oxide production, and mitochondrial activity, sunlight plays a foundational role in human biology – we NEED daily sun exposure. The chronic avoidance of sunlight not only leads to widespread deficiencies but also disrupts core regulatory systems essential to health and resilience.
I have come to understand and therefore argue that sunlight is not a health hazard to be feared, but rather a metabolic input to be managed. Drawing from current peer-reviewed research, this is my attempt to outline the key biological functions modulated by sunlight, explaining how modern exposure patterns misalign with our physiology, and offering practical strategies to reintegrate safe, consistent sun exposure into daily life. In doing so, I am challenging the dominant sun-phobic paradigm and making the case for sunlight as an essential component of human health.
Circadian Rhythms: The Light-Dark Cycle as a Metabolic Blueprint
One of the most critical functions of natural sunlight is its role in regulating the circadian rhythm. This internal biological clock governs the sleep-wake cycle, hormone secretion, body temperature, and metabolic processes on an approximately 24-hour cycle (Roenneberg and Merrow, 2016). Light, particularly blue light in the 460–480 nm range found in morning sunlight, serves as the primary environmental cue or "zeitgeber" to synchronise this clock via our suprachiasmatic nucleus – the master clock of our bodies.
When natural light exposure is consistent, the circadian system operates in harmony with external day-night cycles. In contrast, limited sunlight exposure, especially in the morning, can delay circadian phase and impair melatonin production at night, leading to sleep disorders, mood imbalances, and metabolic dysregulation (Wright et al., 2013). Exposure to artificial light at night (ALAN), particularly from screens and LED bulbs, further exacerbates circadian misalignment by inhibiting nocturnal melatonin release (Cho et al., 2015).
Research has linked circadian disruption to increased risks of obesity, insulin resistance, cardiovascular disease, and even certain cancers (Bass and Takahashi, 2010). Morning sunlight, on the other hand, improves sleep quality, enhances mood via serotonin regulation, and optimises metabolic functioning by aligning internal rhythms with the solar cycle (LeGates, Fernandez and Hattar, 2014).
Simply put, sunlight doesn’t just illuminate our environment allowing us to see - it actually entrains our very physiology and governs our wake / sleep patterns which are critical for optimal health outcomes.
Vitamin D: Beyond Bone Health
Perhaps the most well-known benefit of sunlight is its role in vitamin D synthesis. When UVB rays strike the skin, they convert 7-dehydrocholesterol to previtamin D3, which is then hydroxylated in the liver and kidneys to form active 1,25-dihydroxyvitamin D (Holick, 2007). This hormone-like compound is essential not only for calcium absorption and bone mineralisation but also for immune regulation, muscle function, and inflammation control.
Vitamin D deficiency is now a global health concern, affecting an estimated 1 billion people worldwide (Palacios and Gonzalez, 2014). This epidemic has coincided with increased sunscreen use, urbanisation, indoor work culture, and misinformation around sun safety. Low vitamin D status has been associated with autoimmune diseases (e.g. multiple sclerosis, type 1 diabetes), respiratory infections, depression, and even mortality (Bikle, 2014).
Although oral supplementation can mitigate deficiency, endogenous synthesis via UVB exposure remains the most efficient and natural method of achieving adequate levels. A 10- to 30-minute exposure to midday sun, depending on latitude, skin pigmentation, and season, can produce thousands of international units (IU) of vitamin D (Holick, 2004). Sunscreens with SPF 30 or higher reduce vitamin D synthesis by more than 95% (Matsuoka et al., 1987).
Thus, sunlight is not just beneficial - it’s biologically necessary.
Nitric Oxide, Cardiovascular Health and Solar Blood Pressure Regulation
Beyond vitamin D, sun exposure plays a direct role in cardiovascular regulation through the release of nitric oxide (NO) from skin stores. When UVA light penetrates the skin, it triggers the photolysis of nitrate and nitrite compounds, resulting in the release of NO into the bloodstream (Liu et al., 2014). Nitric oxide is a vasodilator - it relaxes the inner lining of blood vessels, lowering blood pressure and improving circulation.
This mechanism helps explain epidemiological findings showing lower rates of hypertension and cardiovascular disease in sunnier regions, independent of physical activity or diet (Weller et al., 2020). NO also plays roles in mitochondrial efficiency, immune modulation, and neurotransmission. Its release through sun exposure constitutes a rapid, non-vitamin D-dependent benefit of UV light that is often overlooked in discussions of solar health.
Modern avoidance of sun exposure may contribute to the increasing prevalence of hypertension and metabolic syndrome, conditions that are both strongly tied to endothelial function and vascular tone. Integrating modest, daily doses of UVA-rich sunlight, particularly in the morning, may offer cardiometabolic benefits beyond those attributable to vitamin D alone.
Melanin, Adaptation, and the Myth of Solar Fragility
Another adaptive feature modulated by sun exposure is melanin production. Melanin is a pigment produced by melanocytes in the epidermis in response to UV radiation. Its primary function is photoprotection because it absorbs harmful UV rays and reduces DNA damage by converting them into harmless heat (Solano, 2020).
While melanin has often been framed as a passive sunscreen, it is also an active part of immune surveillance. Its production stimulates local antioxidant systems, facilitates apoptotic clearance of damaged keratinocytes, and reduces oxidative stress in the skin (Slominski et al., 2015). This means that gradual, consistent exposure to UV radiation effectively "trains" the skin, increasing its resistance to sunburn - a phenomenon sometimes referred to as building a “solar callus”.
In contrast, when the skin is deprived of sunlight for months and then suddenly overexposed during a vacation or heatwave, it lacks these adaptive defences. This leads to sunburn, which is not a result of sunlight per se, but of poor solar conditioning. The myth that sunlight is inherently harmful arises from this mismatch between how we live and how we evolved.
Immune Modulation and Inflammation Control
Sunlight plays a nuanced role in regulating immune activity. While excessive UV exposure is known to suppress certain immune responses, controlled and regular sunlight exposure supports immune resilience. UVB-induced vitamin D production enhances the function of T cells and macrophages, while sunlight itself influences dendritic cells and cytokine signalling in the skin (Norval and Halliday, 2011).
More interestingly, low-level UV exposure can reduce chronic inflammatory states by modulating nuclear factor kappa B (NF-κB) and interleukin signalling (Yin and Agrawal, 2014). These shifts are associated with decreased incidence of autoimmune diseases in populations with higher habitual sun exposure, including lower rates of multiple sclerosis, type 1 diabetes, and inflammatory bowel disease (van der Mei et al., 2001).
Furthermore, some research suggests that exposure to certain wavelengths of solar radiation may prime the skin’s microbial environment, influencing not only the skin microbiome but also systemic immune responses (Skabytska et al., 2020).
As immune dysfunction and chronic inflammation underpin many modern metabolic disorders, the immunoregulatory effects of sunlight deserve wider attention.
Mitochondrial Biogenesis and Red/Near-Infrared Light
Sunlight also provides red (600–700 nm) and near-infrared (NIR) light (700–1100 nm) that penetrates deeply into tissues. These wavelengths influence mitochondrial function through a process called photobiomodulation (Hamblin, 2017). Specifically, NIR light stimulates cytochrome c oxidase in the electron transport chain, enhancing ATP production, reducing oxidative stress, and activating mitochondrial biogenesis.
The implications are broad: improved energy metabolism, faster recovery from injury, enhanced skin health, and neuroprotective effects. Studies have shown that red/NIR exposure reduces inflammation in the brain, improves cognition, and even extends lifespan in animal models (Zhang et al., 2019). These regenerative and anti-inflammatory effects operate independently of vitamin D and present an entirely separate mechanism by which sunlight supports metabolic health.
Artificial light lacks these beneficial wavelengths - especially those required for deep tissue penetration. This means that time spent outdoors, even in the shade or in indirect sunlight, provides unique cellular inputs not replicable with standard indoor lighting.
Endocrine and Neurochemical Responses: Beyond Melatonin
Sunlight exposure also influences the endocrine system, particularly the release of hormones tied to mood, appetite, and stress resilience. Morning sunlight exposure suppresses melatonin, a hormone that facilitates sleep, and boosts serotonin levels, which regulate mood and impulse control (Lambert et al., 2002). This not only improves alertness but has been linked to lower risks of seasonal affective disorder (SAD) and depression.
In addition, UV exposure stimulates the production of beta-endorphins in the skin, which bind to opioid receptors in the brain and induce a mild euphoric state (Fell et al., 2014). This mechanism is thought to be evolutionarily conserved to encourage sunlight-seeking behaviour, which confers survival benefits through vitamin D production and circadian entrainment.
These endocrine effects reveal that sunlight is not merely a passive environmental factor - it is a dynamic signal that modulates neurology, mood, and behavioural patterns.
Avoidance of sunlight may thus play an underappreciated role in the rise of mood disorders and sleep dysfunction across industrialised societies.
Practical Routines for Safe and Beneficial Sun Exposure
Reintroducing regular sun exposure into modern life requires both strategy and moderation. The goal is to regain the evolutionary baseline of sunlight interaction, meaning steady, daily, low-dose exposure while minimising risk of overexposure and reliance on chemical sunscreens.
A. Build a Solar Callus Gradually
The skin, like muscle or bone, adapts to repeated stress. Begin with short durations of unprotected sun exposure (5–10 minutes) during lower UV index hours, typically before 10:00 and after 16:00, and increase gradually. This allows melanin production and antioxidant systems to scale up, reducing the likelihood of burns or photodamage (Narayanan, Saladi and Fox, 2010).
B. Expose the Right Surface Area
Key hormonal and metabolic responses, including vitamin D and nitric oxide production, require exposure of larger skin surface areas. Forearms, legs, chest, and back are ideal. A face-only approach is insufficient for meaningful physiological effects (Holick, 2004).
C. Optimise Timing and Circadian Effects
Aim for morning sunlight exposure within 30–60 minutes of waking. This anchors circadian rhythms, suppresses residual melatonin, boosts serotonin, and sets the hormonal tone for the day (Wright et al., 2013). Avoid bright artificial light after sunset to maintain melatonin integrity and natural sleep patterns or make use of blue light blocking glasses if needed.
D. Avoid Toxic Sunscreens Where Possible
Most commercial sunscreens contain endocrine-disrupting compounds like oxybenzone and avobenzone, which can interfere with hormone function and cellular signalling (Krause et al., 2012). Use shade, clothing, and mineral-based sunblock (e.g. zinc oxide) instead, especially after your adaptive exposure limit is reached.
E. Supplement Intelligently (Not as a Replacement)
While vitamin D3 supplementation may be necessary in winter or high-latitude regions, it should not be seen as a substitute for sunlight. Supplementation bypasses the many non-vitamin D pathways (e.g. NO, endorphins, circadian cues) that sunlight activates. It is a fallback, not a replacement.
Conclusion
The prevailing fear of sunlight, driven by decades of public health campaigns focused on skin cancer risk, has resulted in a collective deficiency of one of the most foundational inputs to human biology. Consistent, managed exposure to natural sunlight is not just "safe when moderated" - it is essential for optimal metabolic health. From circadian rhythm regulation to cardiovascular health, immune modulation, and mitochondrial function, sunlight acts as a hormonal and metabolic keystone.
The problem lies not in the sun itself, but in our erratic and culturally distorted patterns of exposure. Chronic avoidance followed by intense, unconditioned exposure leads to sunburn and photodamage - giving rise to a false binary of "sun equals skin cancer." In reality, safe, regular sun exposure builds resilience, supports cellular health, and aligns us with the natural environmental rhythms we evolved to thrive within.
As with nutrition or physical activity, the solution lies in intelligent, habitual engagement - not abstinence. Reframing sunlight as a daily nutrient, not a seasonal threat, opens the door to a more grounded, biologically coherent model of health. It's time to come back into the light - carefully, consistently, and with respect for the ancient bond between sun and life!
References:
Bass, J. and Takahashi, J.S. (2010). Circadian integration of metabolism and energetics. Science, 330(6009), pp.1349–1354.
Bikle, D.D. (2014). Vitamin D metabolism, mechanism of action, and clinical applications. Chemistry & Biology, 21(3), pp.319–329.
Cho, Y., Ryu, S.H., Lee, B.R., Kim, K.H., Lee, E. and Choi, J. (2015). Effects of artificial light at night on human health: A literature review of observational and experimental studies. Chronobiology International, 32(9), pp.1294–1310.
Fell, G.L., Robinson, K.C., Mao, J., Woolf, C.J. and Fisher, D.E. (2014). Skin β-endorphin mediates addiction to UV light. Cell, 157(7), pp.1527–1534.
Hamblin, M.R. (2017). Mechanisms and applications of the anti-inflammatory effects of photobiomodulation. AIMS Biophysics, 4(3), pp.337–361.
Holick, M.F. (2004). Sunlight and vitamin D for bone health and prevention of autoimmune diseases, cancers, and cardiovascular disease. The American Journal of Clinical Nutrition, 80(6), pp.1678S–1688S.
Holick, M.F. (2007). Vitamin D deficiency. New England Journal of Medicine, 357(3), pp.266–281.
Klepeis, N.E., Nelson, W.C., Ott, W.R., Robinson, J.P., Tsang, A.M., Switzer, P., Behar, J.V., Hern, S.C. and Engelmann, W.H. (2001). The National Human Activity Pattern Survey (NHAPS): a resource for assessing exposure to environmental pollutants. Journal of Exposure Analysis and Environmental Epidemiology, 11(3), pp.231–252.
Krause, M., Klit, A., Jensen, M.B., Søeborg, T., Frederiksen, H., Schlumpf, M., Lichtensteiger, W. and Skakkebæk, N.E. (2012). Sunscreens: are they beneficial for health? An overview of endocrine disrupting properties of UV-filters. International Journal of Andrology, 35(3), pp.424–436.
Lambert, G.W., Reid, C., Kaye, D.M., Jennings, G.L. and Esler, M.D. (2002). Effect of sunlight and season on serotonin turnover in the brain. The Lancet, 360(9348), pp.1840–1842.
Liu, D., Fernandez, B.O., Hamilton, A., Lang, N.N., Gallagher, J.M., Newby, D.E., Feelisch, M. and Weller, R.B. (2014). UVA irradiation of human skin vasodilates arterial vasculature and lowers blood pressure independently of nitric oxide synthase. Journal of Investigative Dermatology, 134(7), pp.1839–1846.
Narayanan, D.L., Saladi, R.N. and Fox, J.L. (2010). Ultraviolet radiation and skin cancer. International Journal of Dermatology, 49(9), pp.978–986.
Norval, M. and Halliday, G.M. (2011). The consequences of UV-induced immunosuppression for human health. Photochemical & Photobiological Sciences, 10(11), pp.1785–1793.
Palacios, C. and Gonzalez, L. (2014). Is vitamin D deficiency a major global public health problem? The Journal of Steroid Biochemistry and Molecular Biology, 144, pp.138–145.
Roenneberg, T. and Merrow, M. (2016). The circadian clock and human health. Current Biology, 26(10), pp.R432–R443.
Skabytska, Y., Kaesler, S., Volz, T., Chen, K.M., Halata, Z., Hölzel, M., Biedermann, T. and Röcken, M. (2020). Cutaneous microbiota and UV radiation. Dermato-Endocrinology, 12(1), e1726533.
Slominski, A., Zmijewski, M., Pawelek, J. (2015). L-tyrosine and L-dihydroxyphenylalanine as hormone-like regulators of melanocyte functions. Pigment Cell & Melanoma Research, 25(1), pp.14–27.
Solano, F. (2020). Melanin and melanin-related polymers as materials with biomedical and biotechnological applications—cuttlefish ink and mussel foot proteins as inspired biomolecules. International Journal of Molecular Sciences, 21(17), p.6085.
van der Mei, I.A., Ponsonby, A.L., Dwyer, T., Blizzard, L., Simmons, R., Taylor, B.V., Butzkueven, H., Kilpatrick, T. and McMichael, A. (2001). Past exposure to sun, skin phenotype, and risk of multiple sclerosis: case–control study. BMJ, 327(7410), pp.316–321.
Weller, R.B., Wang, Y., He, L. and Feelisch, M. (2020). Pathophysiological consequences of a lack of sunlight: Vitamin D deficiency or beyond? Photochemical & Photobiological Sciences, 19(1), pp.1–14.
Wright, K.P., McHill, A.W., Birks, B.R., Griffin, B.R., Rusterholz, T. and Chinoy, E.D. (2013). Entrainment of the human circadian clock to the natural light-dark cycle. Current Biology, 23(16), pp.1554–1558.
Yin, L. and Agrawal, D.K. (2014). Vitamin D and inflammatory diseases. Journal of Inflammation Research, 7, pp.69–87.
Zhang, R., Mio, Y., Pratt, P., Lohr, N., Warltier, D.C. and Bienengraeber, M. (2019). Near-infrared light protects the heart from ischemia–reperfusion injury by increasing nitric oxide release. Journal of Molecular and Cellular Cardiology, 48(4), pp.474–482.