In the late Nineteenth Century, hospitals were constructed with large floor-to-ceiling windows following Florence Nightingale’s radical treatments of sunlight and fresh air. These health traditions endure with Scandinavian mothers, who line babies up in prams to nap outside year-round to stimulate immunity and sleep quality. There is an inherent bliss in lounging in the sun on a cloudless day, allowing the warmth to saturate your clothes, engulf your entire being, amping up serotonin while lowering cortisol levels. However, since the advent of skin cancer in the U.S., when we already spend much less time outside than our ancestors, we are routinely advised to avoid the sun, slather our skin in ultraviolet (UV) blocking cream, and minimize our exposure with clothing. But have we gone too far? A large portion of adults in the U.S. are already severely vitamin D deficient, about 35-40 percent (Sizar, 2022; Jordan et al., 2022), even though it is regularly added to milk and other foods to “fortify” our diet. When on average, depending on location and skin tone, just 15-40 minutes of daily sun with a good portion of skin exposed during Spring and Summer can supply enough vitamin D to prevent a deficiency (Burchell et al., 2020; Sizar, 2022). While the role of vitamin D deficiency with SARS-CoV-2 is being unfurled, the theory of melatonin production stimulated by sunlight needs attention.

Sunlight boosts immune system beyond vitamin D

A review of 28 studies confirms that vitamin D deficiency correlates with worse outcomes of Covid-19, and having optimal levels prior to, or supplementing early in infection, could result in less severe disease; however, some of those studies reveal vitamin D supplementation for those already infected did not reduce hospital stays with moderate to severe disease (Jordan et al., 2022; Murai et al., 2021). A report in Nature shows a correlation between Covid-19 surge dates and latitude, separate from temperature and humidity—the longer the days with more light exposure, the less cases of Covid-19, and the shorter days, the higher surges of Covid-19. While early optimal vitamin D levels may reduce disease severity, an article in Frontiers in Cardiovascular Medicine (Gorman and Weller, 2020) finds UV light independent of vitamin D reduces deaths from Covid-19. Another study in the British Journal of Dermatology shows UVA exposure in the U.S., which is not responsible for vitamin D3 synthesis, correlates with a 29 percent decrease in the mortality rate ratio (MRR) per 100kJ m-2 of daily increased exposure, and they were able to replicate these results in two other countries (Cherrie et al., 2021). The authors conclude that as “UVA does not appear to act in viricidal suppression of SARS-CoV-2, the reduction in the observed risk might be the result of behavioural change or a biological pathway” (Cherrie et al., 2021). These data seem to suggest that sunlight gives us something else other than vitamin D that is beneficial to our immunity. Some researchers propose that this component is melatonin.

Melatonin is a strong antioxidant

Melatonin synthesizes in the pineal gland at night when we are exposed to darkness, which is why it is known as the “dark hormone.” However, nearly 400 times melatonin is detected in the gut and in many extrapineal tissues of the body at “orders of magnitude higher than circulatory melatonin produced by the pineal gland” (Minich et al., 2022; Zimmerman and Reiter, 2019). Researchers theorize that extrapineal melatonin is synthesized as a response to oxidative stress caused by Near Infrared Radiation (NIR) (Zimmerman and Reiter, 2019) and/or by UV exposure (Izykowska et al., 2008), stimulating production of melatonin right there in the cell where it is needed to eat up those free radicals. Melatonin is a fierce antioxidant and free-radical scavenger—one molecule of melatonin can eat up to 10 reactive oxygen species (ROS) and reactive nitrogen species (RNS) (Tan et al., 2006), while most other antioxidants mop up only a few (Tan et al., 2006; Zimmerman and Reiter, 2019).

Melatonin protects against Covid

Similar to vitamin D levels with Covid-19, Loh and Reiter (2022) explain that “the presence of adequate melatonin before or at the time of infection critically influences the timely suppression of viral infection” and also disease progression. This suppression of viral infection helps recover from Covid-19 and perhaps minimizes the likelihood of developing long Covid, especially seeing as melatonin is also involved in the “modulation of inflammatory and immune cytokines, directly impacting both systemic and acute anti-inflammatory properties” and “both vitamin D and melatonin orchestrate many of their functions, especially related to redox status, at the level of the mitochondria” (Minich et al., 2022). Having vitamin D and melatonin ready and primed before coming into contact with SARS-CoV-2 could help mitigate that inflammation and avoid the horrible acute and long-term effects some have suffered from Covid.

Supplementation with melatonin needs further evaluation

Like many adults are vitamin D deficient, they are probably low on melatonin as well, especially as serum levels decrease inversely with age after spiking in early childhood (Minich et al., 2022). Another reason why children may be so adept at dealing with illnesses like Covid. U.S. melatonin supplement sales were $821 million in 2020, doubling since 2017 (Minich et al., 2022), and while people seem to be rushing to add this antioxidant to their diet, more studies need to be done on how melatonin is produced outside of the pineal gland and if it is best acquired through sunlight or through supplementation. If we know that low levels of vitamin D and low levels of melatonin correspond with poorer outcomes of Covid-19, and sun exposure synthesizes vitamin D, if it also synthesizes extrapineal melatonin, then we need to understand if acquiring it through oxidative stress or through supplementation is the best mechanism to increase it. Sunlight is free, and if melatonin is produced by NIR, we can achieve this even through clothing and nontreated glass, without being exposed to harmful UV rays. Beyond the evidence, we have always known inherently that fresh air and sunlight is good for us. Florence Nightingale recognized this in her patients. And if it’s good for the plants and the animals, then why not us? Perhaps it is something inexplicable, a connection to nature or the feeling of warmth, or perhaps it is our serotonin and cortisol levels optimizing, or maybe sunlight has multiple pathways that stimulate our immune system, melatonin being just one of them. Perhaps it is no mistake that the intricate design and balance of our bodies are connected to our environment, that everything we need to thrive is right here, ready to soak up.

*This article does not constitute medical advice.


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Helen Stead

Helen Stead writes evidence-based nonfiction, dystopian fiction, and is passionate about medical freedom. She earned a Ph.D. in English and Creative Writing from the University of Tennessee, an M.F.A from the University of Missouri, Kansas City, worked in medical settings through two degrees, and has edited several nationally-distributed literary journals.