Feature By Lisa Heschong
A Second Revolution in Light and Health
There is growing evidence that a second revolution in light and health is brewing. The first revolution started about 25 years ago as we began to learn how daily light exposure impacts circadian health: that light exposure at night can cause sleep disruption, while bright light during the day can promote alertness and robust circadian rhythms. We learned that blue wavelengths were particularly potent in signaling the circadian system, and that time of day of exposure mattered a great deal in terms of outcome. People in the lighting industry started to use unfamiliar medical terms like “melatonin” and “cortisol,” and acronyms like SCN and ipRGC. This emerging knowledge ushered in new product concepts, such as “dynamic lighting” and “human-centered lighting design” and eventually formed the basis for a new IES document, RP-46-25: Recommended Practice: Supporting the Physiological and Behavioral Effects of Lighting in Interior Daytime Environments.
The second revolution now afoot may have an even more profound impact on the luminous environment and where we focus our priorities for better human health. New biological pathways, independent of vision and 24-hour circadian rhythms, are being investigated. Fueled by new scientific capabilities, access to massive health databases, and fresh ideas about how light interacts with living tissue, researchers are proposing radical new hypotheses about the relationship of light to human health.
At the largest scale, epidemiological studies of large populations are providing growing evidence that more light during the day results in better overall health. A recent series of studies looking at the long-term health outcomes of tens of thousands of people included in the UK Biobank database have found that the 25% of adults with the brightest light exposure during the day had the fewest psychiatric diagnoses,1 least cardiovascular disease,2 lowest cancer rates, and lowest mortality rates over the course of several decades. Despite fears of increased skin cancer, more sunlight exposure for this UK population even predicted lower skin-cancer rates.3
While the light exposure metrics used in these studies are far too generalized to support any conclusions about the specific characteristics or timing of “bright light,” they are strongly suggestive that there is something about “more bright light during the day” that is protective against many adverse health outcomes. An obvious guess might be that “more bright light during the day” is a result of more time spent in daylight.
For decades, researchers assumed that the many health benefits of sunlight exposure could be attributed to higher levels of Vitamin D, as measured in a blood sample. However, when multiple trials of Vitamin D nutritional supplements showed no similar benefits, the theory began to shift to assuming that perhaps there were other sunlight-linked mechanisms at work. Could high levels of Vitamin D be better thought of as a biomarker of greater sunlight exposure?4
To put these findings in context, a recent pilot study looked at indoor daylight exposure during regular office hours. Thirteen older diabetic adults were recruited to work for two weeks in a corner office. In randomized order, the room was flooded with daylight from corner windows one week, while the windows were blocked with opaque blinds the other week. The subjects were tested for detailed metabolic indicators before, during, and after each week. After 4.5 days of exposure, the test results showed subtle, but significant, improvements in metabolic health indicators.5 Again, the study provides no characterization of optimum dosage; only comparing an office environment lit solely with overhead luminaires to one also with large daylit windows. Even so, it was remarkable to detect measurable changes in metabolic health after spending only one work week in a daylight office.6
In the past decade or so, medical devices have been proliferating for sale on the Internet promising to use red and/or near-infrared light (NIR) to treat a wide array of maladies. The use of such devices applied directly to the body is often referred to as photobiomodulation, or low-level red-light therapy. A recent survey of the field described applications ranging from hair growth, wound healing, head trauma, cancer treatments, and eye health. The range of clinical uses would seem to make NIR therapy a miraculous cure-all, but there is very little research exploring the underlying biological mechanisms, or limits on safe and effective dosage. The same review also expressed concern about “consumer-grade devices whose commercial expansion outpace translational validation.”7
Research has also lagged in delineating normal environmental NIR exposures. To date, only a handful of studies have examined the health effects of ambient level exposures of NIR in indoor environments. Two recent studies stand out:
A 2024 study tested 151 undergraduates from the University of British Columbia. In randomized order, each worked for one hour in a lab cubicle illuminated with normal LED white light, and the same condition supplemented with NIR. The total NIR radiant energy was gauged to be about halfway between normal outdoor NIR levels and that of incandescent lighting indoors. The subjects reported no perceptible differences between the two conditions. However, after one hour of NIR exposure, the researchers detected significant improvements in both cardiovascular and emotional function, which persisted for at least another hour.8 Thus, the study demonstrated that ambient levels of NIR irradiation can have demonstratable physiological and psychological effects.
Another research group at University College London has been conducting a series of experiments, first on animals and then humans, to understand the effects of ambient NIR exposure from sunlight, incandescent, and LED sources. The group has documented NIR transmission deeper through human tissue than previously assumed, and improvements in general health outcomes from small dosages of NIR to various parts of the body. The team leader is a professor of ophthalmology and neurology and has thus used retinal health tests as a sensitive indicator of metabolic impacts. In 2025, the team published the remarkable study, “Longer wavelengths in sunlight pass through the human body and have a systemic impact which improves vision.”9
NIR transmission “through the human body” may seem improbable to many, but many organic materials are translucent to a variety of NIR wavelengths. Likewise, much of our clothing is also semi-transparent to NIR, typically transmitting 60 to 80% of the original source energy. The denser the fabric, the darker the color, and the more layers, the less the transmission, as shown in Figure 1.10
Together these studies and others suggest that NIR exposure may be an important component of a healthy indoor environment. However, over recent decades, indoor NIR exposure has been greatly reduced via two major technological changes:
a reduction in the use of incandescent light sources, including candles and fires, which naturally emit NIR; first in favor of fluorescent sources and more recently in favor of LED sources
growing use of low-e (low-emissivity) coatings on windows which narrow the spectrum of daylight that passes through them.
The change in electric lighting sources has sparked a controversy across the Internet as to whether LEDs should be reformulated to produce more substantial NIR output. Similarly, a controversy is arising as to whether energy-efficient windows should be reconsidered to allow more of the solar spectrum to pass through.
It is only recently that people have begun to consider the potential health consequences of reducing our indoor exposure to solar wavelengths beyond the visible spectrum. It is not only NIR that is a potential concern but also very short wavelengths up into the ultraviolet range.
Beginning in the 1980s, and accelerating in the 2000s, North American homes and workplaces began to be fitted out with new, more energy-efficient windows featuring low-e coatings. These transparent coatings, largely formulated from silver oxides, optimize light transmission centered around 550 nanometers (nm) within the human visible range, while reflecting other solar wavelengths, especially in the NIR and ultraviolet ranges, as unnecessary “waste energy.” Figures 2(a) and 2(b) illustrate the spectral transmission of several low-e product lines, some more aggressive than the others. The spectral transmission properties of a given glazing system are also affected by the number of glass layers, tint, thickness, and chemical composition. The net result of current energy-efficient windows, however, has been to exclude 50 to 95% of both incident NIR and UV solar radiation from the inside of buildings.
One of the most concerning consequences of reduced light exposure indoors may be a reduction in childhood visual development. Normal fetal development leads to functional eyes at birth that progress to perfect 20/20 vision within the first few years of life. But something has started to go very wrong with children’s eye development, such that more children are becoming myopic, i.e., nearsighted. Instead of reaching a perfect shape for clear vision, a myopic eye continues growing and elongating, making it ever more difficult to focus far away. In the U.S., 35 to 40% of children now become myopic for the rest of their lives, compared to 2 to 5% a few generations ago. In Asia, the problem is even worse, affecting up to 90% of adolescents in some of the larger cities. Given the fast generational change, this is clearly a disease with an environmental cause.11
Myopia is typically a children’s disease and ceases progressing during late adolescence or early adulthood. Many people quickly jump to the conclusion that the current global “myopia boom” must be related to increased screen time for children. However, one of the first signs of the myopia epidemic was among Inuit tribes in Canada, who the government aggressively moved into westernized homes and schools in the 1940s and 1950s. Thus, these Indigenous people moved from living predominantly outdoors, even during the long twilights of arctic winter, to indoors under incandescent and fluorescent lighting. By the 1970s, opticians were finding that Inuit children were developing myopia at alarming rates, compared to the negligible rates of their grandparents,12 showing that the myopia epidemic was detected long before computers and cell phones became pervasive in modern life.
Since then, epidemiological studies provide clear evidence that children living in rural areas fare better than those growing up in cities. Lower risk for myopia has also been conclusively tied to children who spend more time outdoors, whether in active sports or quiet reading. This suggests that there is something about the visual environment outdoors that is protective of normal eye development. One hypothesis that has received attention is that it is the spectrum of daylight that provides the protective effect, especially via wavelengths outside of the visible spectrum.13 Experiments conducted with both animals and humans, using both NIR and violet light sources, have shown promising results. It is unclear at this point if one or the other type of light is more effective in promoting optimum vision in children, or if perhaps they work in concert, as a daily symphony.
Many pieces of the puzzle are still missing, but we are beginning to fill in a framework suggesting that light, and especially the natural patterns of daylight, has a more profound influence on our long-term health than previously thought. Of course, sunlight has long been considered a force for human health, with roots in the medical practices of ancient civilizations such as Egypt, India, Greece, and Rome. In the 1850s, Florence Nightingale, founding mother of modern nursing, showed that sunlight exposure sped wound recovery and reduced contagious diseases among soldiers fighting in the Crimean war. Throughout the 19th century, sunlight was a recognized treatment for the global scourges of tuberculosis and rickets. In 1903, Niels Finsen was awarded the Noble Prize in Medicine for the discovery that ultraviolet-light exposure improved tuberculosis recovery and that smallpox scaring could be reduced with red-light exposure.
Today, scientists are beginning to delve deeper into the molecular mechanisms at work and to map their evolutionary history across species. Projects have also been launched to collect detailed light-exposure data from diverse groups of people across continents to better understand what real-life light exposure looks like.14 Soon, perhaps, we will be able to provide a more precise prescription for daily light exposure to keep people healthy. Until that point is reached, the best current advice may be to simply spend more time outdoors in unfiltered daylight.
THE AUTHOR
Lisa Heschong, Fellow IES, was a founding principal of the Heschong Mahone Group and a licensed architect for 30 years. She is an IES Medal recipient and has served on the IES Board of Directors, the IES Daylight Metrics Committee, and as chair of the IES Medals Committee.
1 Angus C. Burns et al. “Day and Night Light Exposure Are Associated with Psychiatric Disorders: An Objective Light Study in >85,000 People.” Nature Mental Health, vol. 1, 2023.
2 Daniuel P. Windred et al. “Brighter Nights and Darker Days Predict Higher Mortality Risk: A Prospective Analysis of Personal Light Exposure in >88,000 Individuals.” Proceedings of the National Academy of Sciences of the United States of America, vol. 121, no. 43, Oct. 2024.
3 Andrew C. Stevenson et al. “Higher Ultraviolet Light Exposure Is Associated with Lower Mortality: An Analysis of Data from the UK Biobank Cohort Study.” Health & Place, vol. 89, Sept. 2024.
4 Richard B. Weller. “Sunlight: Time for a Rethink?” Journal of Investigative Dermatology, vol. 144, no. 8, Aug. 2024.
5 Jan-Frieder Harmsen et al. “Natural Daylight during Office Hours Improves Glucose Control and Whole-Body Substrate Metabolism.” Cell Metabolism, vol. 38, no. 1, Jan. 2026.
6 Ioannis G. Lempesis and Frank A. J. L. Scheer. “Illuminating the Influence of Natural Daylight on Human Metabolism.” Cell Metabolism, vol. 38, no. 1, Jan. 2026.
7 Pooja Shivappa et al. “From Light to Healing: Photobiomodulation Therapy in Medical Disciplines.” Journal of Translational Medicine, vol. 23, 2025.
8 Charlotte M. Roddick et al. “Effects of Near-Infrared Radiation in Ambient Lighting on Cognitive Performance, Emotion, and Heart Rate Variability.” Journal of Environmental Psychology, vol. 100, Nov. 2024.
9 Glen Jeffery et al. “Longer Wavelengths in Sunlight Pass through the Human Body and Have a Systemic Impact Which Improves Vision.” Scientific Reports, vol.15, no. 1, July 2025.
10 Aamer Saleem et al. “Near Infrared Transmission through Various Clothing Fabrics.” Journal of Textile Science & Engineering, vol. 3, no. 2, 2013.
11 Kevin W. Houser, Lisa Heschong, and Richard A. Lang. “Buildings, Lighting, and the Myopia Epidemic.” LEUKOS, vol.19, no. 1, Jan. 2023.
12 Jos J. Rozema et al. “Reappraisal of the Historical Myopia Epidemic in Native Arctic Communities.” Ophthalmic and Physiological Optics, vol. 41, no. 6, Sept. 2021.
13 Lisa Heschong, Richard A. Lang, and Shruti Vemaraju. “New Dimensions of Daylight and Health: How Opsin Biology May Inform Lighting Standards in the Near Future.” Lighting Research & Technology, vol. 57, no. 6–7, 2025.
14 Johannes Zauner, Steffen Hartmeyer, and Manuel Spitschan. “LightLogR: Reproducible analysis of personal light exposure data.” The Journal of Open Source Software, vol. 10, no. 107, Mar. 2025.