Using Sunlight to Optimize Health | Roger Seheult | AFP 48

The following is based on a conversation with Michael Snyder, Roger Seheult, an Assistant Clinical Professor in medicine at Loma Linda University and at University of California, Riverside.

How Does Near-Infrared Light Penetrate Deep Human Tissue and Enhance Mitochondrial Function Across the Body?

Near-infrared (NIR) radiation from natural sunlight (specifically wavelengths around 700 to 900 nm, with peak penetration around 850 nm) can pass through clothing, skin, and up to 30 cm of tissue. Unlike visible or ultraviolet light, NIR photons penetrate deep into the body to reach cellular mitochondria.

At the cellular level, NIR light optimizes the electron transport chain, the final pathway where oxygen accepts four electrons to generate water and ATP. In high-stress or diseased states, incomplete electron transfers yield reactive oxygen species (ROS) such as superoxide or hydroxyl radicals, which cause mitochondrial self-damage. NIR light reduces the activation energy required for Marcus electron transfer within these enzymatic complexes, allowing electrons to transfer efficiently and safely. This process boosts ATP production and neutralizes oxidative stress through mitochondrial melatonin and endogenous antioxidant enzymes.

Furthermore, through a mechanism known as the abscopal effect, stimulating mitochondria with NIR light in one area of the body induces systemic intercellular signaling that upregulates ATP output and cellular repair in distant, unexposed tissues (such as the retina).

Why Does Natural Sunlight Provide Systemic Health Benefits That Oral Vitamin D Supplements Cannot Replicate?

While ultraviolet B (UVB) light triggers the synthesis of vitamin D in the skin, vitamin D is primarily a biomarker of sun exposure rather than the sole driver of sunlight’s systemic benefits. Epidemiological studies consistently show that higher latitude and lower solar radiation correlate with increased mortality from cardiovascular disease, cancer, and respiratory infections, independent of vitamin D levels or socioeconomic class.

Sunlight delivers a continuous electromagnetic spectrum consisting of:

• Ultraviolet (UV): Triggers localized vitamin D synthesis, nitric oxide release, and systemic metabolic signaling.
• Visible Light: Regulates circadian rhythms via retinal pathways to the brain’s habular and occipital regions.
• Near-Infrared (NIR): Comprises over 50% of solar photons, directly charging cellular mitochondria across the body to lower systemic oxidative stress.

Oral vitamin D supplementation only addresses target nuclear receptors; it fails to replicate the mitochondrial energy restoration, anti-inflammatory cascades, and metabolic benefits (such as immediate improvements in insulin sensitivity and triglyceride clearance) provided by full-spectrum solar radiation.

How Have Modern Lighting, Architectural Glass, and Indoor Lifestyles Created a “21st-Century Health Scurvy”?

Modern environments isolate humans from the natural solar spectrum, creating a state of chronic light deficiency:

Environmental Factor	Historical / Natural Condition	Modern / Artificial Condition	Biological Impact
Spectral Composition	Full-spectrum blackbody radiation (Sunlight/Incandescent) containing balanced visible and NIR light.	Narrow-band LED lighting emitting only narrow visible light devoid of NIR frequencies.	Retinal photoreceptors and systemic cells are forced to expend vast amounts of ATP without the supporting NIR wavelengths necessary to rebuild mitochondrial energy.
Architectural Glass	Standard single-pane glass permitting NIR transmission into indoor spaces.	Energy-efficient Low-E (Low Emissivity) windows engineered to block all infrared light.	Completely strips indoor environments of restorative NIR wavelengths, trapping occupants in an infrared-deprived state.
Lifestyle & Habits	Majority of time spent outdoors engaged in agricultural or outdoor activity.	Over 93% of time spent indoors under LED lights behind Low-E glass.	Creates chronic mitochondrial fatigue, elevated baseline oxidative stress, and increased vulnerability to metabolic and infectious diseases.

Much like the 18th-century sailors who suffered from scurvy due to diets lacking raw fruit with vitamin C, modern indoor populations suffer from a chronic deficiency of natural, full-spectrum light.

Cleaned Transcript

Jeroen Schreel: Welcome to Apple Finch Pudding, your gateway into the world of science. Today’s scientist is Roger Seheult, a professor at Loma Linda University and the University of California, Riverside. His work focuses on internal medicine. Welcome, Roger.

Roger Seheult: Thank you so much, Jeroen. It’s great to be here.

Jeroen Schreel: Before we start, do you have a fun science fact for our listeners?

Roger Seheult: A fun science fact! Well, there are some new findings coming out that many people may not be aware of. I work closely with a group in London that has conducted scientific experiments in the field of biology and light. They recently published a paper in Nature Scientific Reports showing that certain wavelengths from the sun can actually penetrate completely through the human body.

Jeroen Schreel: That is very interesting, and that is actually what we want to talk about today: light and sunlight. Before we delve into that—in recent years, you have focused a lot of your work on light therapy and photobiomodulation. Why is that? Was there a specific trigger or event in your life that got you into this research?

Roger Seheult: Yes, there was: COVID-19. I’m a critical care and pulmonary physician, so I take care of patients in the intensive care unit on mechanical ventilators. As you can imagine, I was very busy four or five years ago and had a lot of questions about what we could do better.

A couple of things piqued my interest during the pandemic. We saw that serum vitamin D levels were highly predictive of how patients would fair, but administering vitamin D upon admission didn’t seem to alter their clinical course. I began to suspect that vitamin D was simply a biological marker for something else doing the heavy lifting.

It turned out that “something else” was sunlight. Sunlight provides profound physiological benefits to the human body independent of vitamin D. That realized insight led me down the rabbit hole of investigating the physics and photochemistry of light—topics rarely emphasized in standard medical training.

The second trigger was mitochondrial biology and oxidative stress. As many know, the cellular receptor for the SARS-CoV-2 virus is the ACE2 enzyme. ACE2 functions as a critical modulator of intracellular oxidative stress. When the virus binds and knocks out ACE2, cellular oxidative stress spikes dramatically. That ties directly back into light biology.

Through my work on YouTube with MedCram, I became acquainted with leading luminaries in photobiology. One major issue in modern science is that we operate in isolated silos. Biologists are in one silo, chemists in another, and medical clinicians in a third. Specialized focus is necessary for complex research, but crucial breakthroughs occur when cross-disciplinary insights are shared.

By bringing together light engineers, physicists, astrophysicists, ophthalmological biologists, and clinicians, we began connecting these dots in an entirely new light.

Jeroen Schreel: You really need that interdisciplinary approach. Most people think sunlight is just about vitamin D. Can you explain what happens when light penetrates the body?

Roger Seheult: Let’s look at the physics. The electromagnetic spectrum coming from the sun closely approximates blackbody radiation, emitting a broad distribution of photons at varying energy levels determined by Planck’s law.

1. The Infrared Spectrum: Longer wavelengths, lower frequencies, and lower energy. Over 50% of all solar photons reaching Earth’s surface fall within the infrared spectrum.

2. The Visible Spectrum: Shorter wavelengths and higher energy, ranging from red to violet. This range regulates vision, mood, and circadian rhythms through ocular pathways like the perihabenular nucleus and visual cortex.

3. The Ultraviolet Spectrum: High-energy radiation capable of breaking chemical bonds. UVB converts 7-dehydrocholesterol in the skin into vitamin D precursors, which are subsequently processed by the liver and kidneys into active calcitriol. Beyond vitamin D’s known role in calcium metabolism, vitamin D receptors exist on immune cells throughout the body.

The deep penetration effect comes specifically from long-wavelength near-infrared (NIR) light. Lower frequency waves penetrate objects far more effectively than high frequency ones—much like how bass frequencies from a neighboring car pass right through your vehicle frame.

Professor Glen Jeffery at University College London published a landmark study in Nature Scientific Reports. His team measured the solar spectral irradiance passing through approximately 30 cm of human thoracic tissue. They discovered that near-infrared light centered around 850 nm penetrates skin, deep organs, and clothing over 100 times more effectively than visible wavelengths.

Jeffery then investigated how this affects cellular metabolism. The retina contains the body’s highest concentration of mitochondria, specifically inside color-detecting cone cells. As humans age, mitochondrial ATP production in these cells drops by 40% to 70%, degrading subtle color discrimination.

In a clinical trial, Jeffery exposed subjects to 850 nm NIR light under three conditions:

1. Control (lights off).

2. Full exposure (head and body illuminated).

3. Body-only exposure (head covered with a light-blocking aluminum shield).

The full-exposure group showed statistically significant improvements in retinal color discrimination, proving that NIR upregulates mitochondrial energy output. Remarkably, the group with their heads completely shielded showed similar improvements. Upregulating mitochondria in the torso triggered systemic signaling—known as the abscopal effect—that restored mitochondrial ATP output in distant, unexposed retinal tissues.

Jeroen Schreel: That is incredible. So infrared light penetrates the body, upregulates mitochondrial activity, and triggers this systemic effect. Why do we need to upregulate mitochondrial activity in the first place?

Roger Seheult: Mitochondria are the powerhouses—the cellular batteries—of our body. Mitochondrial dysfunction is central to aging and nearly every chronic disease, including heart disease, neurodegenerative conditions like Parkinson’s and Alzheimer’s, and chronic kidney disease.

Think of a mitochondrion as an internal combustion engine. While it produces work, it generates intense internal heat. Without a proper cooling system, that thermal energy damages and shuts down the engine itself.

In the mitochondrial electron transport chain, oxygen accepts four electrons and four protons at Complex IV to form clean, harmless water (2H2​O). However, if electron transport slips and transfers only one, two, or three electrons, it generates highly reactive oxygen species (ROS) such as superoxide (O2∙−​) or hydroxyl radicals (OH∙). These free radicals inflict immediate oxidative damage on mitochondrial membranes and DNA.

To counteract this, mitochondria possess intrinsic antioxidant cooling systems. For example, subcellular melatonin is produced inside mitochondria at concentrations orders of magnitude higher than in the pineal gland, alongside protective enzymes like superoxide dismutase, catalase, and glutathione.

In chronic conditions like diabetes, hypertension, and severe viral infections like COVID-19, baseline oxidative stress overwhelms these native systems, destroying mitochondrial capacity. When SARS-CoV-2 inactivates the membrane-bound ACE2 enzyme, oxidative stress surges uncontrollably. Patients with pre-existing metabolic disease already had “hot-running engines” near the redline; the viral insult pushed their systems into total overheating and failure.

Near-infrared light exposure directly reduces this oxidative load and restores efficient mitochondrial ATP production across human tissue.

Jeroen Schreel: What about oral antioxidants on product packaging—do they help, or are they ineffective?

Roger Seheult: Certain oral compounds do offer measurable clinical utility. For instance, N-acetylcysteine (NAC) has been demonstrated in randomized, double-blind, placebo-controlled trials to decrease the clinical severity of influenza and COVID-19 by supporting intracellular glutathione synthesis.

However, if NIR light truly acts as a primary drive for mitochondrial health, we should see clear epidemiological evidence linking sunlight exposure to lower disease burden.

And we do. Historically, 100 years ago, hospitals were explicitly engineered with massive windows and outdoor solariums because physicians observed that open-air solar exposure accelerated recovery times. Modern hospital studies confirm that patients assigned to beds nearest to windows have shorter average lengths of stay than those in interior beds.

During the 2020 pandemic surge in Europe, researchers examined environmental correlations across latitudes. While temperature and humidity showed no statistical correlation with infection spikes, latitude demonstrated a striking 73% correlation. Outbreaks progressed directly from northern latitudes (Finland, Sweden, Norway) downward, hitting southern areas like Greece last. When evaluating solar ultraviolet exposure as a proxy for total sunlight, that correlation rose to 93%.

Richard Weller, a dermatologist at the University of Edinburgh, evaluated COVID-19 mortality relative to ambient Ultraviolet A (UVA) exposure. Across the United States, England, and Italy—spanning diverse socioeconomic populations—higher regional solar irradiance consistently correlated with significantly lower COVID-19 mortality.

The most definitive proof comes from randomized controlled clinical trials. In a double-blind Brazilian study, 30 hospitalized COVID-19 patients were randomized to wear a flexible therapy jacket fitted with 940 nm NIR LED emitters. The intervention group received low-level NIR illumination (2.9 mW/cm2) for just 15 minutes once per day.

Despite the intervention group being clinically sicker at baseline, NIR therapy delivered dramatic, statistically significant improvements across all endpoints:

• Faster stabilization of oxygen saturation (SpO2​).

• Increased spirometric lung capacity and respiratory muscle strength.

• Faster normalization of heart and respiratory rates.

• A reduction in average hospital stay from 11.7 days down to 8.0 days.

A simple 15-minute daily dose of invisible near-infrared light slashed nearly four full days off hospital admissions.

Jeroen Schreel: That is incredible. How severe is our modern light deficiency?

Roger Seheult: It is a massive, unrecognized health crisis.

In 1992, an Environmental Protection Agency (EPA) survey found that Americans spent 92% to 93% of their lives indoors. That was before smartphones, high-speed internet, streaming media, and remote delivery services. Today, indoor confinement is even more severe.

Furthermore, our indoor environments have been engineered to exclude natural light:

1. Artificial Lighting Bans: To conserve electricity, regulatory mandates have largely replaced broad-spectrum incandescent bulbs with Light Emitting Diodes (LEDs). Standard incandescent bulbs act as blackbody radiators, emitting visible light alongside generous amounts of near-infrared wavelengths. LEDs emit narrow spectral peaks designed purely for human visual perception while completely eliminating near-infrared wavelengths. For the first time in human history, we are bathing our eyes and skin in visible light without the co-present, protective NIR photons needed to support mitochondrial metabolism.

2. Energy-Efficient Low-E Glass: Modern building codes mandate Low-Emissivity (Low-E) window glass. Low-E coatings are designed to reflect thermal infrared wavelengths to lower air conditioning loads. Consequently, even when sitting inside a brightly sunlit modern office, the beneficial near-infrared spectrum is filtered out.

We have engineered a environment where we stay indoors 93% of the time, illuminated by light devoid of near-infrared wavelengths, behind glass that blocks the sun’s natural NIR spectrum.

Jeroen Schreel: What practical steps can people take? How long do we need to be outside?

Roger Seheult: The single best solution is to get outside into natural light.

Unlike UV radiation—which requires specific solar angles and clear skies to synthesize vitamin D—near-infrared light penetrates cloud cover, shade, and winter atmospheres effortlessly. Long-wavelength NIR photons pass through light clothing and wide-brimmed hats.

Data from Glen Jeffery’s laboratory demonstrates that 15 to 20 minutes of daily natural outdoor light exposure is sufficient to charge mitochondrial ATP production. Even on freezing winter days, the sun’s NIR radiation is active. If solar radiation has enough thermal energy to melt rooftop snow in sub-freezing ambient temperatures, it possesses more than enough energy to penetrate human tissue and stimulate biological pathways.

A major 20-year prospective study in Sweden tracking 30,000 women demonstrated that those with the highest sun exposure had the lowest all-cause mortality, lowest cardiovascular mortality, and lowest cancer mortality. In fact, non-smokers who avoided the sun had an all-cause mortality rate equivalent to smokers with high sun exposure. Richard Weller recently replicated these findings across the massive UK Biobank cohort.

If getting outside during the workday is impossible, adding a traditional 60-watt incandescent desk lamp to an LED-lit office provides broad-spectrum continuous light, yielding measurable improvements in mitochondrial health and visual performance.

As photobiologist Bob Fosbury famously stated: “The lack of natural sunlight is the scurvy of the 21st century.”

Jeroen Schreel: How rapidly do these biological benefits occur?

Roger Seheult: Some physiological adaptations occur almost immediately. A joint study by Oxford University and Leiden University evaluated 10,000 subjects, comparing metabolic panels against local meteorological data. They discovered that higher ambient sunlight exposure over the preceding seven days directly correlated with improved insulin sensitivity and lower fasting serum triglycerides.

Other adaptations are long-lasting. In laboratory settings, brief exposures to broad-spectrum light restored cellular ATP production for up to six weeks post-exposure, likely by upregulating nuclear gene expression for key mitochondrial transport proteins.

The therapeutic potential in clinical medicine is vast. A triple-blind, randomized controlled trial published in 2024 evaluated broad-spectrum NIR light therapy across general intensive care unit (ICU) patients. Irrespective of their primary diagnosis—whether septic shock, major trauma, or post-operative recovery—patients receiving NIR therapy experienced a 30% reduction in total ICU length of stay and demonstrated greater physical muscle strength upon discharge.

I recall a 15-year-old oncology patient who developed an invasive, life-threatening pulmonary fungal infection following intensive chemotherapy. After losing his left lung to surgery, his remaining right lung developed severe fungal infiltrates. Intubated on high-pressure BiPAP support with persistent fevers and failing vitals, his care team estimated he had 48 hours to live.

His final request was simply to go outside. Hospital staff wheeled his bed, ventilators, and infusion pumps out into the courtyard. He spent four to five hours in direct sunlight daily. Within 72 hours, his fevers resolved and his white blood cell count normalized. Follow-up CT imaging revealed that the dense fungal infiltrates in his remaining lung had cleared. He made a full recovery and remains healthy today.

Over 150 years ago, Florence Nightingale—the pioneer of modern nursing—observed without modern diagnostic machinery that fresh air and direct sunlight were essential for healing. Modern molecular physics is now validating her clinical observations. If you are battling chronic disease, metabolic dysfunction, or seasonal illness, take the fight outside.

Jeroen Schreel: You are the very first person who has truly explained the underlying science of sunlight in a way that proves its critical importance. Thank you so much, Roger.

Roger Seheult: Thank you, Jeroen.

Jeroen Schreel: This was Apple Finch Pudding. I want to thank Roger Seheult for his incredible insights. Join us next time for our next deep dive into science!

Relevant Papers

• Jeffery et al., 2025. Longer wavelengths in sunlight pass through the human body and have a systemic impact which improves vision. Scientific Reports, 15: Article 24435; https://doi.org/10.1038/s41598-025-09785-3
• Jeffery & Barett, 2025. LED lighting undermines visual performance. Preprint: https://doi.org/10.21203/rs.3.rs-6540877/v1
• Cherrie et al., 2021. Ultraviolet A radiation and COVID-19 deaths in the USA with replication studies in England and Italy. British Journal of Dermatology, 185: pp. 363-370; https://doi.org/10.1111/bjd.20093
• Lindqvist et al., 2014. Avoidance of sun exposure is a risk factor for all-cause mortality: results from the Melanoma in Southern Sweden cohort. Journal of Internal Medicine, 276: pp. 77-86; https://doi.org/10.1111/joim.12251
• Stevenson et al., 2024. Higher ultraviolet light exposure is associated with lower mortality: An analysis of data from the UK biobank cohort study. Health & Place, 89: Article 103328; https://doi.org/10.1016/j.healthplace.2024.103328
• Pereira et al., 2023. Cardiopulmonary and hematological effects of infrared LED photobiomodulation in the treatment of SARS-COV2. Journal of Photochemistry and Photobiology B: Biology, 238: Article 112619; https://doi.org/10.1016/j.jphotobiol.2022.112619
• Neto et al., 2024. Photobiomodulation therapy (red/NIR LEDs) reduced the length of stay in intensive care unit and improved muscle function: A randomized, triple-blind, and sham-controlled trial. Journal of Biophotonics 17, Article e202300501; https://doi.org/10.1002/jbio.202300501
• Noordam et al., 2019. Associations of Outdoor Temperature, Bright Sunlight, and Cardiometabolic Traits in Two European Population-Based Cohorts. The Journal of Clinical Endocrinology & Metabolism, 104: pp. 2903-2910; https://doi.org/10.1210/jc.2018-02532