Sunlight: Medical Villain to Metabolic Victor

The core features of being human determine our metabolic health: how we eat, sleep, breathe, hydrate, move, think, and connect with others. Our circadian rhythm regulation and exposure to sunlight also play critical, but often neglected metabolic roles. There is no life without the sun. It is 150 million kilometers away, but it powers every life form on earth: humans are no exception. As humans, we are a diurnal species, evolved to be in the light during the day and the dark at night. However, many of us have that backwards, much to our metabolic chagrin. In 2007, the International Agency for Research on Cancer (IARC), part of the World Health Organization (WHO) classified circadian disrupting night shift work as a Group 2A probable carcinogen, increasing the risk of breast, prostate, and colorectal cancers.¹ A 2025 study in the Journal of the American Medical Association (JAMA) found that light exposure at night is a significant risk factor for coronary artery disease, elevating the risks of heart attack, stroke, atrial fibrillation and heart failure.² And in 2024, a study of over 88000 found that those adults with dark days and bright nights had a higher risk of premature mortality.³ When we fight nature, we lose. 

But how and what have we lost? By emphasizing the hazards of too much sun exposure with the potential for premature aging and skin cancer, we have overlooked the more common metabolic benefits we can reap. First, let us understand the different light energy parts from the sun. Sunlight is divided into three main categories: infrared (IR), visible and ultraviolet (UV). 

UV light is made of high energy, short wavelength (100-400nm) light that can penetrate down to the skin dermis, making it clinically useful in the treatment of inflammatory skin conditions, such as psoriasis. UV light is further subdivided into UVA, UVB and UVC. The UVC light from the sun has potent germicidal activity, but is blocked by the ozone layer. Of the UV light that reaches us, 95% is UVA which helps to make nitric oxide (NO) in our skin, critical to our cardiovascular  health. NO is not only our most important vasodilator, relaxing blood vessels and lowering blood pressure, but it is also the most potent anticoagulant we make, helping to prevent heart attacks and strokes.⁴ The remaining 5% of UV light is UVB light. This is highest at noon and converts a cholesterol derivative in our skin, 7-dehydrocholesterol, to pre vitamin D3.⁵ Because of this reaction, vitamin D is known as the "sunshine vitamin." Vitamin D is critical to our health, playing a key role in our metabolic, neurologic, immunologic and bone health. In children, vitamin D deficiency causes growth restriction and rickets; in adults it causes osteopenia, osteoporosis and an increased risk of fracture. Given its powerful preventative effect on children's health, milk has been fortified with vitamin D since the 1930's.⁶ However, our time indoors, inadequate consumption of vitamin D rich foods, elevated BMI, heavy sunscreen use, aging, seasonal exposure, skin pigmentation and latitude have left as many as 50% of us at risk for vitamin D deficiency.⁵ Numerous studies have shown associations between vitamin D deficiency and multiple cancers, cardiovascular disease, diabetes, autoimmune disorders, dementia and depression.⁶ Regular testing and supplementation are key for optimal health. On bloodwork, those who have 25-hydroxy vitamin D [25 (OH)] levels below 20 ng/mL have a deficiency, where 20 and 30 ng/mL suggests insufficiency. Sufficient 25(OH) levels are 30-60 ng/mL.⁵ 

The visible spectrum (400-750nm) is vital for our circadian rhythm regulation. By synchronizing our eating with daylight, we can work with our hormones rather than against them. This is a low cost, high-impact metabolic upgrade by optimizing glucose, insulin and melatonin. The early morning sun, high in blue light energy, tells our brain’s pineal gland to stop making our sleep hormone, melatonin. It's time to be alert. With melatonin turned off and the sun up, this is our eating window. We are more insulin sensitive during the day, meaning we do NOT need as much insulin to metabolize our food. The less insulin our pancreas secretes to metabolize food, the less fat we make. Getting outside after meals helps to lower our blood glucose because sunlight also boosts insulin sensitivity. Again, this is a free, simple, non-pharmacologic intervention to improve our metabolic health. The evening sun is high in red light, raising melatonin which makes us sleepy. The nocturnal rise in melatonin also has a role in regulating glucose levels.⁷ As melatonin increases, it inhibits insulin release from the pancreas. This makes us more insulin resistant in the evenings, an important adaptation to make sure our hungry brains have enough glucose while we sleep. This is a signal to stop eating because it’s time to rest: the fasting window begins. Being more insulin resistant at night is the reason why a big meal late at night results in higher blood glucose. The farmers were right. The best time for a big meal is noon, when we are the most insulin sensitive.⁸ 

There has been an explosion of research on the metabolic impact of red and infrared energy. Exposing even small regions of skin to red light has a whole body metabolic impact. In a remarkable experiment, Glen Jeffery demonstrated that red light has the power to improve our metabolism by reducing blood glucose.⁹ Test subjects who drank a standard glucose solution and then received 15 minutes of 670nm red light on a patch of skin had an almost 30% reduction in blood glucose elevation two hours later. Light directly influences our metabolism because mitochondria are light sensitive. The capacity of light to create healing is known as photobiomodulation (PBM). 

By improving mitochondrial function and ATP production, red and infrared light can improve our metabolism and reduce blood glucose. Infrared is the lower energy, longer wavelength (750nm-1mm) part of the spectrum. Similar to the UV end of the spectrum, IR is also divided into three broad categories: near (NIR), mid (MIR) and far (FIR). FIR is commonly used in IR saunas, boosting cardiovascular health, detoxification and athletic performance.¹⁰ The NIR spectrum of sunlight deeply penetrates our tissues by many centimetres to directly provide energy for our mitochondria, improving their function through ATP and antioxidant production.¹¹ We evolved with the metabolic benefits of the full light spectrum from sunlight. Fire light and our older incandescent bulbs are also rich in these beneficial longer wavelengths, mimicking sunlight. 

Just as we can choose real food or junk food, we are becoming aware of the metabolic implications of avoiding real light and our recent exposure to “junk light:” LEDs. Our LED lit world lacks the long-wavelength red and infrared energy found in sunlight. This may be contributing to our epidemic of metabolic dysfunction found among 93% of us. LED light bathes us in a very narrow portion of the visible blue light spectrum. Yes, even those “warm yellow” LED lights are blue. In the 21st century, we now spend over 90% of our time indoors, starving our light hungry bodies of the metabolic benefits found in full spectrum light. Our retinas are the most mitochondria-dense tissue, giving it the highest metabolic rate and fastest pace of aging in our body.¹²In a clever experiment involving office workers exposed to eight hours of LED light, half had their light supplemented by an incandescent bulb placed in a lamp on their desk for two weeks. In visual testing after the incandescent exposure, those workers had a significant improvement in their ability to detect colour contrast, lasting for months.¹³ Simply adding back an incandescent bulb to your work environment has a powerful impact. 

This information is meant to be empowering, not overwhelming. You can choose pieces to modify: 

Within an hour of waking, could you get outside for morning light on a regular basis? This and the evening sun help to anchor our circadian rhythm. Even if you can just pause and get some sun on your face before you go to work, it’s worth doing. 

Could you pack a bigger lunch and eat a smaller supper if you want to lose weight? If the weather permits, could you eat outside or go for a walk after you eat? Could you change your lightbulbs or sit next to a window when you work? Would you wear blue light blocking glasses in the evening, turn down lights and limit screens? 

We are solar powered. The sun helps us make what we need, from vitamin D to nitric oxide, and it regulates our metabolism, sleep and hormones, too. Given these positive metabolic effects of sunshine, it’s little wonder that a study among Swedish women demonstrated that sun avoidance resulted in the same mortality as smoking cigarettes!¹⁴ Yes, the non-smokers who avoided the sun had a life expectancy similar to smokers who received the most sun. Take home message: don't smoke and don’t fear the sun. 

May your nights be dark and your days be bright with full spectrum light. Get outside and enjoy the sun. Just don’t get burned.

References: 

1) Straif K, Baan R, Grosse Y, et al. Carcinogenicity of shift-work, painting, and fire-fighting. Lancet Oncol. 2007;8:1065–1066. 

2) Windred DP, Burns AC, Rutter MK, Lane JM, Saxena R, Scheer FAJL, Cain SW, Phillips AJK. Light Exposure at Night and Cardiovascular Disease Incidence. JAMA Netw Open. 2025 Oct 1;8(10):e2539031. 

3) D.P. Windred, A.C. Burns, J.M. Lane, P. Olivier, M.K. Rutter, R. Saxena, A.J.K. Phillips, & S.W. Cain, Brighter nights and darker days predict higher mortality risk: A prospective analysis of personal light exposure in >88,000 individuals, Proc. Natl. Acad. Sci. U.S.A. 121 (43) 

4) D. Liu, B.O. Fernandez, A. Hamilton, et al. UVA irradiation of human skin vasodilates arterial vasculature and lowers blood pressure independently of nitric oxide synthase J. Invest. Dermatol., 134 (7) (2014), pp. 1839-1846 

5) Kaur J, Khare S, Sizar O, et al. Vitamin D Deficiency. [Updated 2025 Feb 15]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan 6) Holick MF. Sunlight, ultraviolet radiation, vitamin D and skin cancer: how much sunlight do we need? Adv Exp Med Biol. 2014;810:1-16. PMID: 25207357. 

7) Mulder H, Nagorny CL, Lyssenko V, Groop L. Melatonin receptors in pancreatic islets: good morning to a novel type 2 diabetes gene. Diabetologia. 2009 Jul;52(7):1240-9. 8) Carrasco-Benso MP, Rivero-Gutierrez B, Lopez-Minguez J, Anzola A, Diez-Noguera A, Madrid JA, Lujan JA, Martínez-Augustin O, Scheer FA, Garaulet M. Human adipose tissue expresses intrinsic circadian rhythm in insulin sensitivity. FASEB J. 2016 Sep;30(9):3117-23. 

9) Powner MB, Jeffery G. Light stimulation of mitochondria reduces blood glucose levels. J Biophotonics. 2024 May;17(5):e202300521. 

10) Hussain J, Cohen M. Clinical Effects of Regular Dry Sauna Bathing: A Systematic Review. Evid Based Complement Alternat Med. 2018 Apr 24;2018:1857413. doi: 10.1155/2018/1857413. 

11) Jeffery, G., Fosbury, R., Barrett, E. et al. Longer wavelengths in sunlight pass through the human body and have a systemic impact which improves vision. Sci Rep 15, 24435 2025. 

12) Campello, L. et al. Aging of the retina: molecular and metabolic turbulences and potential interventions. Annu. Rev. Vis. Sci. 1146/annurev-vision-100419-114940 (2021) 13) Barrett, E.M., Jeffery, G. LED lighting (350-650nm) undermines human visual performance unless supplemented by wider spectra (400-1500nm+) like daylight. Sci Rep 16, 3061 (2026). 

14) Lindqvist PG, Epstein E, Landin-Olsson M, Ingvar C, Nielsen K, Stenbeck M, Olsson H. Avoidance of sun exposure is a risk factor for all-cause mortality: results from the Melanoma in Southern Sweden cohort. J Intern Med. 2014 Jul;276(1):77-86.276(1):77-86.