Modern Indoor Life Has Replaced Sunlight With a Narrower Light Diet

Rowan Jacobsen frames the current enthusiasm for light therapy as a sign of something larger than a wellness trend. Red light panels, seasonal affective disorder lamps, green light for migraines, ultraviolet treatments for psoriasis, and newer UV devices for systemic conditions can look, in isolation, like unrelated attempts to shine light on problems. They make more sense if light is treated less as an external intervention and more as a set of inputs modern life has removed.

The central premise is simple: every one of those therapeutic wavelengths is already present in sunlight. White sunlight contains ultraviolet, the visible spectrum, and infrared. “Light medicine,” as Jacobsen uses the term, is partly an effort to re-supply wavelengths the human body historically received outdoors but no longer receives reliably indoors.

That matters because light is not merely illumination. Jacobsen describes photons as energy moving at 300,000 kilometers per second. When photons hit the body, some reflect, but much of the energy enters tissue and interacts with molecules. A photon vibrating at a frequency that matches a molecule can be absorbed by that molecule, transferring energy and changing its behavior. Light can act biologically because photons are not symbolic signals; they are energetic events.

Different wavelengths also reach different depths. Ultraviolet light, the highest-energy portion he discusses, is absorbed almost immediately in the epidermis. It can do useful work there, but it can also damage molecules, including DNA. Visible light penetrates farther, through the skin and into subcutaneous tissue. Infrared penetrates farthest. Jacobsen says there are relatively few molecules in the body tuned to absorb infrared photons, so they can move through tissue until they find absorbing targets, many of which are associated with mitochondria.

That is why he began thinking of photons as “photonutrients.” Sunlight, in this metaphor, is not one active ingredient but a “beautiful mezze platter” of energy inputs. Red, green, blue, UV, and infrared are not interchangeable. The body may use different wavelengths in different places, at different depths, and through different mechanisms.

The contrast with indoor light is one of the major claims. A slide comparing full-spectrum sunlight with a modern cool-white LED showed sunlight as a continuous spectrum with a small ultraviolet component, substantial visible light, and a large infrared tail. The LED spectrum, by contrast, was discontinuous, mostly visible, and typically less than 1% infrared. Jacobsen gives the solar spectrum as roughly 53% infrared, UV around 3% to 4%, and visible light around 43% to 44%. Indoors, efficient lighting is designed not to emit energy people cannot see. That means indoor light is “a completely different mix of photons than any living thing has ever encountered until about 20 years ago.”

Intensity is the second difference. A typical indoor office is around 400 lux; the room he was speaking in measured around 600 lux. Outside, he measured about 5,000 lux in the shade of umbrellas, and about 98,000 lux in sun. An overcast day can be around 10,000 lux; a sunny day around 100,000 lux. Human eyes conceal this difference because they can adjust across about a million-fold range of light intensity. But circadian researchers, he says, refer to many indoor conditions as “biological darkness” because they do not register to the body as daytime.

The claim is not that artificial light is useless. It is that a life spent mostly under indoor LEDs supplies a dim and spectrally narrowed version of what the body evolved with. Modern people may not merely be “indoors” more often. They may be running a different light diet.

The decline in outdoor time is historically abnormal in Jacobsen’s account. A chart he showed placed average daily outdoor time at roughly three and a half hours per day in the 1950s and less than an hour per day now. For children, he cited a recent BBC story saying the average child in England gets less than three hours per week outdoors. The point is evolutionary and epidemiological: this level of sun deprivation is new for the species.

Low light exposure is associated, in the material Jacobsen presents, with a long list of diseases: heart disease, stroke, diabetes, Alzheimer’s, colon cancer, breast cancer, respiratory disease, multiple sclerosis, inflammatory bowel disease and Crohn’s, Parkinson’s, metabolic syndrome, obesity, depression, and skin cancer as the exception. More sun, he says, is associated with more skin cancer, but less of many other chronic diseases.

The limits of broad environmental data remain central. Latitude correlations and population-level comparisons can suggest where to look, but they do not prove causation. “Correlation does not prove causation,” Jacobsen says, treating ecological evidence as an opening, not a verdict.

Multiple sclerosis is his example of a light-sensitive disease. He calls it “the sunshine disease,” saying children who do not get sunshine in their first years of life have multiple-times higher rates of MS than children who spend more time outdoors. A visual attributed to Simpson et al., 2011, showed MS prevalence by latitude, with rates rising at higher latitudes. Jacobsen describes the chart as logarithmic, with prevalence ranging from around 5 to 800 per 100,000, and says rates in Scandinavia can be 200 to 400 times higher than in low-latitude countries. He says it is believed to be because of light.

He also invokes the “Scots’ Paradox,” a term shown in a review article by Oliver Gillie, to describe higher chronic disease rates and worse mortality in Scotland than in Southern England despite otherwise similar demographics. Researchers, in his telling, think light is probably part of the explanation.

For the United States, the claim is more provisional. Jacobsen says a large study using National Cancer Institute county-level UV exposure data and NASA satellite data was forthcoming and unpublished. He had seen the data, he said, but could not give specifics. His summary was that it followed the same curve: more UV, more melanoma, but lower rates of the other diseases he had been discussing.

The stronger evidence, in his hierarchy, comes from individual-level studies. He highlights the Melanoma in Southern Sweden cohort, which tracked 30,000 Swedish women for 20 years and collected self-reported sunbathing habits while monitoring disease outcomes. The study set out to identify melanoma risk factors. As expected, women most actively exposing themselves to sun had higher melanoma rates. Unexpectedly, he says, they had significantly lower rates of cancer, cardiovascular disease, and diabetes.

A chart attributed to Lindqvist et al., 2014, compared survival among sun avoiders, moderate sun-exposure participants, and the most active sun-exposure participants. After 15 years, 92% of sun avoiders were alive compared with 96% of sun seekers. Jacobsen describes that as twice the mortality rate among sun avoiders and says the study adjusted for usual confounders such as exercise, diet, and socioeconomic status.

The study he treats as especially important is a UK Biobank analysis using wearable light sensors. A PNAS article title shown on screen read: “Brighter nights and darker days predict higher mortality risk: A prospective analysis of personal light exposure in >88,000 individuals.” The visible significance summary said the study captured roughly 13 million hours of data from light sensors worn by about 89,000 people over 40, and found that brighter nights and darker days predicted higher premature mortality after accounting for sociodemographic and lifestyle factors.

Those exposed to the most artificial light at night were about 21% more likely to die from any cause, Jacobsen says. In the day, the pattern reversed: those exposed to the most light were about 34% less likely to die from any cause. After adjustment for confounders, he says the high-light group remained 17% less likely to die from any cause than the low-light group.

A daylight-mortality slide attributed to Windred et al., 2024, showed the largest visible improvement not at the very top of the exposure distribution, but in the move out of the lowest-light group. The lowest daylight-exposure category was assigned a relative all-cause mortality risk of 100. The next category, 50% to 70%, was 90. The 70% to 90% group was 84, and the 90% to 100% group was 83.

The interpretation is not that everyone needs maximal sun. The finding Jacobsen emphasizes is the jump from the lowest half of daylight exposure to a modestly higher category. “You don’t want to be in that low light category,” he says. The public-health problem, as he presents it, is not failure to optimize sunlight exposure perfectly. It is the health penalty of living at the low end.

Jacobsen’s argument depends on holding two points at once: sunlight increases skin-cancer risk, especially under some exposure patterns, and sunlight may reduce risk for other major causes of death. He treats skin cancer as the central constraint in any recommendation to get more light.

A slide asked, “What about skin cancer?” and contrasted annual global mortality figures: cardiovascular disease at 20 million, cancer at 10 million, respiratory disease at 8 million, dementia at 3 million, and melanoma at 60,000. Jacobsen’s point is that melanoma receives intense public attention, partly because skin-cancer messaging has been effective, but it is a small cause of death relative to cardiovascular disease, cancer, respiratory disease, and dementia. Morbidity still matters, he says, but public-health recommendations must consider what moves the largest mortality burdens.

The better-supported risk hierarchy in his talk begins with phenotype and burns. The highest-risk group, in his description, is people with very light skin and very light hair, many moles, and a history of sunburns. For them, the rules should differ. Asked later whether sunlight’s only negative is skin cancer, he said skin cancer is the big one, but not the only consideration: lupus can be exacerbated by sunlight; audience members mentioned rosacea and Parkinson’s-related melanoma risk.

The exposure pattern matters. Intermittent sun exposure—being indoors most of the year, then taking a sunny vacation and burning—raises melanoma risk about 1.6 times, according to Jacobsen. Everyday sun exposure, by contrast, “actually lowers your risk of melanoma by 5%,” in his phrasing. His interpretation is that the dangerous pattern is not regular, moderate light but shocking skin that has not adapted to sun.

The more provocative part concerns ultraviolet A and B, and Jacobsen keeps it visibly provisional. UVB is the higher-energy ultraviolet band that causes sunburn and can directly break DNA bonds. UVA is lower-energy than UVB but still higher-energy than visible light. Older sunscreen strategies focused heavily on UVB because UVB visibly caused sunburn and could damage DNA directly.

A recent National Cancer Institute study, as Jacobsen describes it, separated UVA and UVB exposure using NASA satellite data. He said he had not seen that done before. According to his account, once UVB was removed from the equation, the highest quartile of UVA exposure had three and a half times the melanoma rate of the lowest UVA quartile. When UVB was examined without UVA, the highest UVB quartile had less than half the melanoma rate of the lowest quartile.

Jacobsen says it “looks like” UVB may be protective because it produces protective compounds, while UVA may be the larger melanoma problem. He calls the finding potentially capable of flipping 50 years of recommendations, while saying he is curious to see where the discussion goes.

A dermatologist in the audience pressed exactly this point. She noted that sunburn increases melanoma risk and sunburn is UVB-based. Jacobsen agreed that a history of burns is associated with significantly higher melanoma rates and that UVB causes burn. His reply was that UVA and UVB usually arrive together. A sunburn history may mark a combined exposure in which UVB caused the burn while UVA contributed to melanoma-relevant damage. He added that melanocytes sit low enough in the skin that relatively little UVB reaches them, estimating that about 70 photons of UVA reach that depth for every one photon of UVB. He labeled the conclusion “just a guess” and “super early.”

Sunscreen came up in the same exchange. Jacobsen’s short version was that U.S. sunscreens are about to improve because the FDA had cleared the way for Bemotrizinol, or BEMT, an organic sunblocking ingredient he said had been used in Europe and Asia for decades but had not been legal in the United States. Older U.S. sunscreens “kind of sucked,” in his view, though they improved over time. He expects newer ones to be better.

The practical implication is not “avoid sunscreen.” It is more specific: protect high-exposure, high-value skin areas, avoid burns, recognize individual risk, and do not confuse the danger of intermittent overexposure with the effects of regular modest exposure. The strongest cautions in the talk are conventional ones: burns matter, burn history matters, and some skin types need stricter rules. The UVA/UVB inversion is the emerging claim, not the settled one.

Rowan Jacobsen treats vitamin D as the first and most familiar sunlight mechanism, but not the complete explanation. The older model was straightforward: sunlight makes vitamin D; sun exposure also causes skin cancer; therefore people should avoid sun and take vitamin D pills. He says that model did not work as hoped.

Vitamin D supplements are clearly useful for people with very low levels—he gives below 16 nanograms per milliliter as an example where supplementation is warranted. But for people with normal vitamin D levels, he says supplementation has not produced the broad benefits many expected. He cites New York Times headlines about vitamin D pills failing to prevent fractures or other conditions, and a 2022 New England Journal of Medicine opinion piece shown on screen with the title “VITAL Findings — A Decisive Verdict on Vitamin D Supplementation.” Jacobsen summarizes that piece as essentially saying to “throw in the towel” and stop broadly prescribing vitamin D.

His explanation is that skin-made vitamin D is embedded in a much larger photochemical system. A pathway diagram attributed to Slominski et al., JID, 2025, showed many metabolites produced from a proto-cholesterol molecule by sunlight, temperature, and skin enzymes. D3—the compound people take in a pill—is only one product. The chart, he says, contains about 25 or 30 vitamin-D-like compounds, “kind of like a cousin of vitamin D,” many of which show similar effects when tested.

Those compounds, in his description, are anti-cancer, anti-inflammatory, anti-proliferative, and involved in damage repair. UV can damage DNA and other molecules, but it also prompts the skin to produce compounds that upregulate repair systems, help fix DNA damage, and inhibit proto-cancer cell proliferation through vitamin D receptors. Sunlight creates both the challenge and the local response: UV hits skin, creates potential damage, and also stimulates molecules that help repair and contain it. Those molecules then circulate and may produce similar effects elsewhere in the body.

He also describes other mechanisms: direct dermal immune modulation, nitric oxide mobilization, neuroendocrine effects, and circadian rhythm regulation. Sunlight can make nitric oxide in the skin, dilating blood vessels and lowering blood pressure. Bright days and dark nights support circadian rhythms and sleep. A slide summarized improvements in vasodilation and blood pressure, metabolism and ATP, antioxidant and mitochondrial function, inflammation and immune tolerance, cognition and mood, damage repair and healing, and circadian rhythms and sleep.

Jacobsen compares the overall pattern to exercise. Exercise is not good for the body in the immediate moment in a simple sense: it stresses cells, accelerates metabolism, creates damage, and tears muscle. The benefit comes because the body responds to a manageable stress by repairing damage, extinguishing “smoldering fires,” and building back better. That is hormesis.

This framing distinguishes moderate exposure from both deprivation and abuse. Too little light may leave repair systems unstimulated and circadian signals weak. Too much light, especially burning exposure, can overwhelm protective systems. The dose and pattern are the argument.

Jacobsen uses specific wavelengths to show why “more light” is too crude a recommendation. Green light, near-infrared light, and ultraviolet light have distinct effects and risks.

Green light is his simplest mechanistic example. He describes checking himself into a sleep lab at the University of Arizona, where researchers exposed him to different light regimes across 24 hours while tracking his brain waves and cognitive performance. Bright white light produced his best performance on cognitive tasks. Red light “sucked for everything,” as he put it. Green light did not produce peak performance, but he says he felt calm and good under it, and his EEG resembled patterns seen in people meditating or trying to meditate.

The proposed mechanism involves the eye’s cone cells. Human eyes have cones sensitive to red, green, and blue. Jacobsen says green cones send smaller electrical signals to the brain, producing less “synaptic potentiation”—less electrical storm—than blue or red photons. He connects that to lower neuroinflammation and to research using green light to reduce migraines and post-surgical pain. A paper shown on screen was titled “Green Light Exposure Elicits Anti-inflammation, Endogenous Opioid Release and Dampens Synaptic Potentiation to Relieve Post-surgical Pain.”

He then links that to green spaces. Plants use blue and red light for photosynthesis and reflect green and infrared. Any green space, in his description, supplies a large dose of green and infrared photons together. That may be part of why forest bathing feels good, though he frames that as an explanatory possibility, not a proven claim.

Near-infrared light is important because it penetrates deeply. Jacobsen says most cells are hit by infrared photons when a person is outside, and near-infrared seems to make mitochondria work “a little bit better,” producing more ATP for the same amount of fuel or oxygen. He refers to a study in aged Drosophila melanogaster in which near-infrared light increased ATP, extended lifespan, and improved mobility. He also mentions a hospital study, without remembering where it was, in which patients exposed to sunlight had hospital stays about 30% shorter.

A photograph attributed to Bob Fosbury showed infrared light passing through a hand; Jacobsen says the image showed light coming through tissue, with veins absorbing it. Another Fosbury infrared photograph of England in summer showed leaves and grass “bouncing” infrared. The point is not that infrared is a miracle cure, but that it is abundant outdoors and mostly missing indoors.

Ultraviolet is the most contested wavelength. Jacobsen calls it “the big scary one” because high-energy UV can break DNA bonds and scramble molecules. Melanin acts as a shield in the upper skin, blocking UV rays and letting enough through to do useful work. UV is required for vitamin D synthesis, but his broader claim is that UV also initiates a network of protective metabolites and immune effects. UVB is especially important in his account because it triggers vitamin-D-related protective chemistry. UVA may be more melanoma-relevant, though he repeatedly treats that as an emerging and unsettled area.

That difference matters in winter. In high-latitude winters, people can get some blue light in their eyes and perhaps a little infrared through clothing, but they get little or no UVB. Above about 35 degrees latitude, Jacobsen says, the sun is too low in winter to provide much UVB. He describes this as a deficiency, particularly for people in places such as Vermont, where winter lasts for months.

His metaphor is deliberately odd: UVB is “the liver of light.” Like liver in a predator’s kill, he says, it is not something one would want constantly, but it is the high-nutrient portion. The question, for him, is how to get the “liver” in midwinter without the dangers of tanning beds.

Jacobsen situates today’s light-medicine revival in a longer medical history. In the early 1900s, physicians learned that rickets could be cured by sunlight because sunlight made vitamin D. Niels Finsen won the Nobel Prize in 1903 for using sunlight to treat cutaneous tuberculosis. That opened a period of heliotherapy, especially in Alpine clinics, where patients were treated with sunlight and fresh air.

Historical images from Dr. A. Rollier’s clinics in Leysin, Switzerland, showed patients and children receiving sun treatments outdoors. Jacobsen describes children attending “school in the sun,” taking classes outside year-round and skiing in minimal clothing under supervision. Hospitals also built roof gardens and wheeled patients outdoors as part of recovery.

He also notes that the movement overreached. A slide showed “baby cages” in New York City in the 1920s and 1930s: babies placed in wire cages suspended from apartment windows for fresh air. Jacobsen’s comment was simple: “Do not try this at home.”

His account of heliotherapy’s disappearance is not that sunlight was shown to be useless. It is that antibiotics and other drugs arrived in the 1940s and 1950s, making sunlight-based healing seem old-fashioned and inconvenient. Pills and drugs were easier, and medicine moved on.

Outdoor treatment is now reappearing, in his telling. He cites a BBC story about a London intensive care ward opening on a rooftop, where machines can be plugged in outdoors and patients can receive care in open air. He says patients are “loving it” and that hospital stays are again going down.

His broader forecast is that medicine and personal health will increasingly include “light diets” and curated “lightscapes.” He describes a small wearable device, shown clipped to a shirt in a mobile-app interface, that tracks which photons hit a person over the day, including infrared and ultraviolet, and gives feedback through a phone. Health practices often change only after measurement becomes possible. If people can measure light composition and exposure, they may start to see relationships they previously ignored.

This is where he places newer light devices. He opposes tanning beds as the answer because they are clearly associated with higher melanoma rates. But he does not dismiss people who seek them out, especially in northern climates. In Vermont, he says, friends who use tanning beds are not irresponsible teenagers chasing a tan. They are middle-aged people who say they “need it,” crave it instinctively, use it briefly, and feel better afterward. His conclusion is that the response should not be to call them idiots, but to create safer and more precise alternatives.

He mentions Solius, a prototype device he tested, as one possible direction. If modern people are deficient in specific wavelengths, especially UVB in winter, targeted light therapies may eventually fill gaps for people who cannot easily get sunlight because of geography, work, culture, clothing, urban form, or medical constraints.

The equity point follows from that. He briefly endorses the “right to light” movement, arguing that many people lack access to good light and that the problem worsens as skyscrapers grow. Access to sunlight and healthy light environments is, in his view, not merely a lifestyle preference but a public-health and equity issue.

Rowan Jacobsen ends with a deliberately modest seven-word summary: “Get sun, not too much, go outside.” The practical advice is moderate, timed, and individualized.

The first practical focus is circadian rhythm. He calls it the low-hanging fruit because bright days and dark nights can produce large health effects without requiring risky midday sun. To explore this himself, he rented a 1960s Airstream in the Sonoran desert for a month and lived without artificial lighting. He says he got bored, but slept well, had strong daytime energy, and found sunset unexpectedly therapeutic. Without artificial light to interrupt the transition, he watched light drain slowly from the sky over about three hours. The gradualness made him feel “very, very happy” and made ordinary indoor life—steady artificial light until lights-out—seem abrupt and unnatural.

Morning light is especially useful in his account. It gives the body a timestamp and helps align circadian rhythms with the day. It is also relatively gentle for skin because when the sun is low, light passes through much more atmosphere, filtering out many high-energy photons. Sunrise and sunset light are richer in orange, red, and infrared, with less of the high-energy ultraviolet that creates trouble. Morning light will not make vitamin D, he says, but it also will not cause much trouble, and it may prepare skin for later exposure by ramping up damage-repair systems in anticipation of stronger midday UV.

Shade is his other key recommendation. He argues for being outdoors, showing some skin, but not necessarily standing in direct sun. Under a tree in a park, a person receives some UV, much less direct sun, and many photons bouncing through the atmosphere, including green and infrared reflected by vegetation. He calls this a likely human sweet spot: a way to titrate dose while receiving a broad outdoor spectrum.

Winter requires different thinking. Outdoor activity in cold climates is better than staying inside, especially for blue light exposure through the eyes, but clothing blocks most skin exposure and high-latitude winter provides little UVB. Saunas can provide infrared—Jacobsen says any sauna, including a wood-stove sauna, is effectively an infrared sauna—but infrared is only one part of the picture. The missing piece, in his account, is often UVB. He does not recommend tanning beds; he sees a role for better-designed devices where natural exposure is impractical or unsafe.

The question period narrowed the advice rather than overturning it. On skin tone, an audience member asked whether chronic-disease disparities among darker-skinned minority populations might partly relate to melanin blocking sunlight at latitudes farther from the equator. Jacobsen said the question was important and that darker-skinned populations tend to have higher rates of some diseases that may be alleviated by sunlight, including diabetes, heart disease, and certain cancers. More melanin blocks more sunlight, so darker-skinned people need more sun to get the same biological effect. He cited unpublished work by Corey Washington comparing blood pressure and sun exposure in Black and white populations. According to Jacobsen, the study found equivalent blood pressure when exposure was proportional to burn threshold: it took more sunlight in the Black population to achieve the same effect because less was getting through. He then offered a back-of-the-envelope extrapolation: if dark skin requires five to 10 times more sun exposure for the same vitamin D effect, it may require something like that for other sunlight benefits too. An audience member noted that Washington had not been able to get the study published.

Circadian guidance also has limits. Asked about waking with a sun shade rather than blackout curtains, Jacobsen said the answer depends on the person. Circadian advice initially tended to be universal—no light after a certain hour, morning light for everyone—but chronotypes differ. Some people are larks, others owls; some are more sensitive to night light; some benefit more from morning light. He personally likes morning light immediately and said there is some evidence people do better when they shift earlier, though he treated an economic statistic about early risers making more money as possibly reverse causation.

Asked to specify safer morning hours, he said the more intense sun window is generally 10 a.m. to 4 p.m., though season matters. Before that, when the sun is low, light passes through much more atmosphere, filtering most high-energy photons. Morning light is gentle, good for circadian rhythm, and unlikely to make vitamin D or cause trouble. For UVB, he suggested either a device or exposing less chronically overexposed skin rather than making the face and hands “do all the work.” In a deliberately comic formulation, he said the way to get UVB is “either to use a device or to do naked cartwheels at noon for 10 minutes.”

The practical logic is consistent across the talk: avoid burns, avoid extreme deprivation, protect high-risk skin and high-exposure areas, get bright outdoor light early, use shade intelligently, and recognize that “sun exposure” is not one behavior.