October 2025

A Brief History of Natural Light Therapy: From Rickets to Cancer

Joseph Corsini, Ph.D. and Julie Alessandra, MTE

For more than a century, light therapy (termed photobiomodulation) has been used to treat human disease. In the late 1800’s Niels Finsen studied the application of light in therapeutic situations and received the 1903 Nobel Prize for recognizing that exposure to direct sunlight or light of certain wavelengths was able to cure a variety of maladies including devastating skin infections caused by Mycobacterium tuberculosis (lupus vulgaris). At about the same time, Palm (1890) noticed that natural sunlight was curative for patients with the bone condition rickets. This was a re-discovery of observations in the early 1800’s by a Polish physician Jedrzej Sniadecki (see Mozaowski 1939), who noted in a report that sunlight could prevent and cure the prevalent malady rickets. Palm (1890) reaffirmed this observation and over the next 20 years medical researchers established that natural sunlight prevents rickets and determined the ultraviolet (UV) fraction of sunlight was the preventative component (Hess and Unger 1921, Webster and Hill 1925) eventually discovering vitamin D and the pathways by which UVB light generates vitamin D3 in the animal body (reviewed in Holick 2006). At this time, recognition of the need for natural sunlight was utilized by medical practitioners around the world (Eliot and Park 1938). By the 1940’s, however, efficient ways to supplement vitamin D3 in the diet were discovered, leading to adoption of supplementation strategies and abandonment of natural light therapies for treatment of rickets.  

While sidelined in favor of supplements or pharmaceuticals, the modern science of light therapy clearly shows its effectiveness as a treatment for many health conditions. Rosenthal (1984) used light therapy to treat seasonal affective disorder, while Goel et al (2003) reported positive effects in placebo-controlled trials of light therapy on chronically depressed individuals, and in recent years there have been hundreds of studies examining the therapeutic benefits of light for depressive and other mental disorders. The ameliorative effects of natural light on sleep disorders are also well-characterized and long reported (Terman and Terman 2005, reviewed in Faulkner et al 2019).  The effects of natural light on cancer is of great interest, and the link between circadian disruption, natural light exposure, and cancer is being actively explored, with many studies suggesting that disrupted circadian cycles can predispose cells to the effects of mutagens and other oncogenic agents. Specifically, there is evidence for direct interaction between PER (one of the central biological clock genes) and the p53 pathways (central to apoptosis and DNA damage control pathways), as well as for control of telomerase and expression of some cyclins (reviewed in Sulli et al 2019). Some of these studies have been conducted in vivo with transgenic mice, providing direct evidence of these effects in living animals. Influences of light cycles and exposure on the microbiota have also been explored (Beshehsari et al 2019), and while experimental work has not yielded clear connections, labs are testing hypotheses to determine the effects of light and circadian cycles on the microbiota, especially as it relates to metabolic syndrome.  The therapeutic effect of near infrared light (NIR) is also being explored. NIR has been shown to positively affect mitochondrial function in cultured cardiomyocytes treated with toxic compounds (Zhang et al 2009) and similarly microglial cells (Stepanov et al 2022), switching cells from lactate (fermentative) to oxidative metabolism. There is also varied evidence for positive outcomes using near infrared light (NIR) to treat a variety of endothelial conditions (Colombo 2022).

The connection between natural sunlight deprivation and cancer has been long known and largely ignored. It was noticed over a hundred years ago by Hoffman (1915), who observed that indoor workers had an 8-fold higher risk of dying of cancer. Three quarters of a century later, Garland et al (1990) suggested that geographic variation in breast cancer mortality could be linked to differing exposures to solar radiation.  Studzinski and Moore (1995) review the evidence for the role of sunlight exposure in reducing overall cancer risk suggesting that the effect happens through the vitamin D pathways. In 2009 Zhee and colleagues reviewed the literature to assess the effects of sunlight on a variety of non-skin cancers, concluding that the available research shows that chronic sun exposure reduces the risk and improves outcomes for colon, breast, lung, Hodgkin’s lymphoma, and even melanoma. These authors also point out the vitamin D connection and note that others have also invoked the melatonin and folic acid pathways as potential co-mechanisms. Besides the vitamin D and melatonin involvement, mitochondrial involvement is almost certain, as are aspects of the central nervous system and microbiome that influence immune function.  We note while vitamin D is obviously at work in the observed anticancer effects, the situation is complex and the mechanistic aspects of the effects of natural light remain poorly understood.

Another aspect of the natural light and cancer story is the link between mutagenic properties of UVB and skin cancer which led to widespread public health messaging warning people to avoid the sun. While the epidemiological correlation between sun exposure basal cell carcinoma and the more serious but less common squamous cell carcinoma was shown in Schmitt et al (2011) and Green and Olsen (2017), there have been conflicting results in a variety of studies. Similarly, the correlation of sunlight exposure with melanoma is not as obvious as once believed. In addition, the large body of work exploring the effects of UV irradiation in mouse models, while providing insight into the working of the cell and genes that control carcinogenesis, suffer from two overarching problems. The first is that nearly all of those studies were and are conducted with mutant strains of mice (the wild type strains refused to develop melanomas – see Day et al 2017) that have been irradiated with artificial UV light instead of natural sunlight. The second is that mice are short-lived animals that don’t live long enough to develop melanomas naturally, and most human melanomas occur after a decades-long life; this means that the mechanisms that lead to melanomas in the laboratory animals are very likely not the same as those that lead to melanomas in humans. Regardless, it is clear that our natural sun exposure cycles have been disrupted by indoor living that prevents the constant exposure required by our innate protective mechanisms. In addition, our sporadic exposure promotes burning that causes damage to DNA, priming normal skin cells for transformation into cancer cells, especially in the temperate zones.

Furthermore, there is accumulating evidence suggesting that the ubiquitous artificial light in our modern world actually appears to damage mitochondria and promote carcinogenesis, likely through disruption of the vitamin D and pineal melatonin pathways (see following for examples). As early as 2006 a link between breast cancer and artificial light at night was suggested by Stevens, and since then others (Kim 2015; Keshet-Sitton et al 2016; Al-Naggar and Anil 2016; Garcia-Saenz et al 2018) have addressed this possibility specifically in relation to breast and other cancers. In summary, these studies suggest that artificial light at night is a bona fide risk factor for a variety of cancers. In this global study, the authors controlled for a variety of other risk factors (such as air pollution and electricity consumption), finding positive correlations with artificial light exposure and the incidence of a variety of cancers. Mechanistically, these adverse effects are likely connected with the circadian pathways and pineal melatonin, although direct effects on retinal and skin cells may also be involved. To wit, a recent study by McNish et al (2025) experimentally measured the effects of blue light exposure on cultured human dermal fibroblasts, showing that increasing blue light exposure adversely affected mitochondrial function (proton leaking and reactive oxygen species formation). Unfortunately, the authors neglected to do a natural sunlight control, so we have no idea how their results compare to natural sunlight. Despite this omission, the study demonstrates the adverse effects of isolated/artificial blue light on cell and mitochondrial function.

Today, our knowledge of light phenomena in the human body has advanced to the point where we now know the precise quanta of light (the number of photons) required to produce each molecule of vitamin D3 in human skin. We understand at the quantum level the physics of infrared light absorption by water, of UVB light by melanin, and of visible light by retinal in the retina of the eye.  We understand the role of full-spectrum (natural) light in controlling circadian rhythms and sleep patterns. Collectively, this large body of research has led us to understand the importance of full spectrum light that contains all of the sun’s electromagnetic output. So, with this sophisticated understanding of light and its necessary interactions with the human body, why has light therapy, especially full spectrum sunlight, been largely ignored by the western medical establishment? Hamblin and Liebert (2022) asked this question and attribute the omission to two main issues: 1) A lack of large-scale clinical trials, and 2) A lack of a clear mechanism of action. Both of these circumstances are true, but one must ask why there have been no clinical trials and why the mechanisms have not been well characterized. The straightforward answer is that private and public funding on both of these fronts has not been forthcoming. While the foundational science has been good, the use of light, especially sunlight, is still viewed as too simple a solution for these complex health issues (despite the fact that, paradoxically, the mechanism of action for effects of any intervention, including pharmaceuticals, on cells and organisms has been difficult to tease out). Given that we rely on the pharmaceutical industry to fund and perform the majority of clinical trials and there is no motivation for the industry to study natural solutions that are available to us all, it’s unlikely we’ll see clinical trials on these topics unless they are funded through a different avenue.

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