Light

Light shapes hormone balance through the eyes and skin, not only vision. Mainstream textbooks long claimed visible light was not chemically active in animal tissues. Peat cited Otto Warburg and later enzyme studies showing visible light restores respiratory pigment activity, and that blue light inhibits while red light activates respiratory enzymes.

Penetrating red light is possibly the fundamental anti-stress factor for all organisms. The chronic deficiency of such light is, I think, the best explanation for the deterioration which occurs with aging.

— Ray Peat

When I moved from Mexico, first to Montana and then to Oregon in 1966, I became very conscious of how light affects the hormones and the health. (For example, in Montana I experienced an interesting springtime shedding of body hair.) Many people who came to cloudy Eugene to study, and who often lived in cheap basement apartments, would develop chronic health problems within a few months. Women who had been healthy when they arrived would often develop premenstrual syndrome or arthritis or colitis during their first winter in Eugene.

— Ray Peat

"winter sickness" in cloudy climates: absence of bright light can leave Estrogen and Prolactin unopposed by Progesterone, worsening PMS, arthritis, colitis, and fertility problems seasonally. Bright artificial light can regulate hormones; Times Square sparrows have been observed mating in winter under city lights.

Darkness and stress hormones (melatonin, prolactin, TSH) increase retinal sensitivity to injury from light that would otherwise be harmless. Cytochrome oxidase is damaged by stress and blue light and restored by red light, Thyroid, and Progesterone; combined with excess oxygen and too little CO2, blue light exposure further damages the retina.

Practical themes: daylight exposure, Red light and incandescent or halogen sources over blue-heavy LEDs at night, orange lenses to block blue when needed. See Roadmap/08 - Buy list & Life quality improvers, Harm reduction.

Light therapy

Controlled bright light exposure shifts circadian timing and mood; mainstream psychiatry uses it for seasonal affective disorder and some sleep disorders. In a 22-day randomized trial of 22 patients with seasonal depression, daily light treatment cut depression scores by 65.2% with standard 10,000 lux white light and by 76.4% with low-intensity 750 lux blue-enriched light, with no significant difference between the two intensities.

The absence of bright light would create a progesterone deficiency, and would leave estrogen and prolactin unopposed.

— Ray Peat

If the sparrows of Times Square mated in the winter because of the bright lights, it seemed clear that bright artificial light would be helpful in regulating human hormones.

— Ray Peat

Daytime bright light supports Thyroid and opposes darkness-driven prolactin and TSH. See Sleep, Harm reduction.

Incandescent and other light sources

Fluorescent-light exposure induced highly conserved inflammatory and acute-phase-response gene programs across zebrafish, medaka, and hairless mice in skin, brain, and liver. TNF and IL1B signaling drove most of these pathways, which the authors interpret as an ancient vertebrate stress response to artificial light.

Fluorescent room light (<200 lux) in the 8 hours before bedtime cut melatonin between its onset and bedtime by 71.4% compared with dim light (<3 lux) in 116 healthy young adults. See Melatonin. Incandescent lamps heat a tungsten filament until it glows, producing a continuous spectrum rich in red and near-infrared wavelengths; halogen bulbs are a related design. White LEDs often peak in blue, unlike incandescent warmth.

By the 1960s, several studies had been published showing the inhibition of respiratory enzymes by blue light, and their activation by red light.

— Ray Peat

Red light is protective, blue light (or u.v.) is harmful, so wearing orange lenses would be helpful.

— Ray Peat

Blue light inhibited cytochrome oxidase and caused retinal injury in rat experiments. Daylight, incandescent or halogen indoor lighting, and orange lenses are favored over blue-heavy LEDs at night. See Red light for red/NIR light therapy devices and protocols.

Fluorescent lighting and melanoma

In a 1982 case-control study from New South Wales, Australia, fluorescent light exposure at work was associated with about double the melanoma risk in women. Risk grew with years of exposure and was higher in women who worked mainly in offices than in other indoor workers. Exposed women had a relative excess of melanomas on the trunk, and a small group of men showed a similar rise with long exposure. The paper itself asked for caution until more data accumulated.

Later case-control studies mostly did not confirm the finding. No consistent association turned up in a US study of 114 patients, in Nottingham (83 patients), in a UK postal survey (58 patients), or in Perth, Western Australia (337 cases, 349 controls). A Scottish study found no consistent rise with fluorescent lights at home or work, while ultraviolet lamps and sunbeds carried a relative risk of 2.9 (1.3-6.4). A larger study of 583 cases and 608 controls in southern Ontario found risk rising with years of occupational and home fluorescent exposure in men and no consistent association in women, the group in which the original signal appeared.

In spectral measurements of common indoor lamps, fluorescent, quartz halogen, and even tungsten filament incandescent bulbs emitted UVA, UVB, and sometimes UVC, some at intensities of similar magnitude to sunlight. Melanoma commonly develops on relatively shaded skin such as the middle of the back and the inside of the thigh, and people who work outside have a low melanoma incidence in some studies.

Nighttime light and the heart

In 11,071 UK Biobank participants with wrist light sensing then CMR ~3 years later, high nighttime light (>3 lux) versus none associated with left-ventricular concentric hypertrophy and lower contraction fraction (about +2.4% LV mass indexed to height^1.7, +1.5% wall thickness, -1.9% myocardial contraction fraction), plus lower absolute LV strain indices and adverse right-ventricle and left-atrium volume/function measures; dose-response was linear and shorter sleep mediated about 24-49% of most associations. In a larger Biobank sample (n=73,286) followed 8-10 years, high versus no nighttime light (>3 lux) associated with higher incident heart failure (+29%), atrial fibrillation (+15%), myocardial infarction (+24%), stroke (+33%), and CVD mortality (+26%). Shorter sleep statistically mediated a large share of the CMR associations; dark nights and adequate sleep are the practical frame. See Sleep, Heart, Blue light.

Blue light

In mouse eyeball cultures, blue light raised markers of oxidative damage to fats in the light-sensing cells. In mice on a Western diet, 12 weeks of blue light made fatty liver worse and raised PUFA buildup in the liver.

Eyes

Cataracts are lens opacity that blurs vision. Surgery is standard when function fails. Systemic energy, sugar chemistry, and light history shape risk.

Peat's view

Cataracts share the same respiratory defect as heart disease and cancer: too much lactic acid and too little CO2, producing a "swelling pathology" in which the swollen lens turns milky just as the swollen heart beats poorly.

Practice

Ophthalmology for treatment. Support metabolic health and avoid unnecessary UV burn. See Light, Sugar, Aging, Sunlight.

Glaucoma is optic nerve damage with high eye pressure. Medical and surgical eye care lead. Systemic vascular and metabolic health are background factors.

Peat's view (eyes)

Serotonin activates carbonic anhydrase, an enzyme class that converts CO2 to bicarbonate; carbonic anhydrase inhibitors like acetazolamide, used for epilepsy and mountain sickness, are also a long-standing treatment for glaucoma. Ophthalmology monitoring. See Light, Brain, Hypertension, Harm reduction.

Intermediate age-related macular degeneration

Age-related macular degeneration damages central vision through retinal cell loss. Ray Peat connected retinal aging to the same PUFA/iron/light-sensitivity framework covered on Pineal and Taurine.

PUFA, iron, and light damage the retina together

PUFA and excess iron can combine with light exposure to damage retinal tissue, since PUFA reacts readily with both light and free radicals:

Light and stress, especially with excess iron, damage the retina when the cells contain too much PUFA, since these fats react with light and free radicals.

— Ray Peat

A 2026 cell study found DHA (a highly unsaturated omega-3) uniquely triggered iron-dependent oxidative cell death (ferroptosis) in retinal pigment epithelium cells under oxidative stress, while a saturated fatty acid caused minimal damage; blocking the iron-driven death pathway restored cell viability. An earlier study from the same lab exposing ARPE-19 retinal pigment epithelium cells to 500 µM DHA combined with 500 µM hydrogen peroxide for 6-18 hours found significantly reduced cell viability compared to either agent alone, an effect reversed by the ferroptosis inhibitor ferrostatin-1 or the necroptosis inhibitor necrostatin-1.

He also described melatonin and prolactin, the "nocturnal/stress hormones" (see Pineal), as making the retina more sensitive to light damage, tying macular aging to the same darkness/stress framework applied to sleep and circadian rhythm elsewhere on this site:

The nocturnal/stress hormones, especially prolactin and melatonin, make the retina more sensitive to light, and more easily damaged.

— Ray Peat

Taurine specifically was cited in the same article as blocking the retinal-damage effects of excess melatonin and supporting metabolism in the pigmented epithelium (see Taurine for the full quote and citation). Standard ophthalmology care remains necessary. Within this framework, lowering PUFA intake, moderating iron supplementation, and supporting normal light/dark rhythm (see Pineal) are the relevant dietary and lifestyle levers, alongside adequate vitamin A food sources.

Myopia (nearsightedness) is a refractive error in which the eye focuses distant images in front of the retina instead of on it, usually because the eyeball has grown too long from front to back (axial elongation) relative to its focusing power. Myopia has become far more common over the past few decades, especially among children, and researchers increasingly point to environmental drivers such as near work and reduced outdoor light exposure alongside genetics.

Near work and screen time

Extended close-focus activity, reading and screen use among them, is a leading suspected driver of myopia progression in children. A 2024 randomized clinical trial by Xie et al. tested naked-eye 3-dimensional vision training (NVT), a daily 20-minute near-focus exercise, against no intervention in 263 children aged 6 to 18 with mild to moderate myopia across three Chinese hospitals. After 6 months, the NVT group's eyes elongated less than the control group's: axial length increased 0.18 mm versus 0.23 mm, and refractive error worsened less (-0.25 D vs. -0.35 D). Both differences were statistically significant, and no study-related adverse effects were reported.

Defocus and axial length

A 2010 study by Read, Collins and Sander found that the eye actively adjusts its own axial length in response to the direction of optical blur it experiences, a mechanism relevant to how near work and uncorrected refractive error might drive elongation over time. Twenty-eight young adults (14 myopic, 14 with normal vision) wore lenses that induced either myopic defocus (+3 D, image focused in front of the retina) or hyperopic defocus (-3 D, image focused behind the retina) in one eye for 60 minutes, with the other eye left in normal focus. Hyperopic defocus produced a small increase in axial length (mean change +8 ± 14 μm,, while myopic defocus produced a decrease (mean change -13 ± 14 μm,. There was no difference between the myopic and normal-vision groups in how strongly they responded. The authors concluded the visual system can detect the sign of defocus, not just its presence, and pushes axial length in whichever direction would move the retina closer to the focal plane, a bidirectional feedback signal that offers one mechanistic explanation for why sustained near-focus blur could promote progressive elongation.

Unpublished case report

A 2025 OSF preprint by Mario Martinez-Saito reported a single adult subject whose myopia reportedly fell from about -6 D to 0 D over five months using a self-directed regimen of induced myopic defocus and conscious accommodation exercises. This is an unpublished, non-peer-reviewed, single-subject (n=1) report, not a controlled trial, and it should be treated as an anecdote worth further study rather than evidence that the method works in general. It has not been replicated. The RCT and defocus evidence above both point in the same general direction: minimizing sustained close-focus blur (correcting refractive error appropriately, taking breaks from near work) and getting time in outdoor daylight are the interventions with the most support for slowing myopia progression in children. The self-reversal case report is not established treatment and shouldn't be treated as one until it's replicated under controlled conditions.

Myopia is not mostly genetic: lack of bright light exposure reduces retinal dopamine production, and dopamine normally blocks eye elongation during development. Injecting a dopamine-inhibiting drug into chicks' eyes abolished bright light's protective effect, confirming the mechanism.

Clothing

Natural and synthetic fibers respond differently to electricity: in lab tests, cotton, regenerated cellulose fibers, and polyester (PET) each showed distinct electric properties tied to their structure and how much moisture they take up. This page tracks natural-fiber clothing and footwear brands as alternatives to synthetic, petroleum-derived fabric.

Footwear

Natural-fiber footwear, as alternatives to synthetic soles and uppers:

• Earth Runners: barefoot, plastic-free grounding sandals.
• ARES Groundwear Conductive Sneakers: grounded lifting shoes.
• Apex Short: gym clothes without the plastic.
• Alpineville: elastic-free 100% organic merino wool socks.

Grounding

Living skin keeps up its own static field as long as it is breathing, and this field collapses when cyanide stops respiration. Daytime barefoot contact with the earth is enough to get through the night without a grounded bed.

Measured one by one, cotton, regenerated cellulose fibers such as viscose and bamboo, and polyester differed significantly in their electric properties, in line with their chemical structure and how much moisture each takes up.

Natural Fabrics	Toxic Fabrics
Leather	Polyester
Silk	Viscose
Cashmere	Nylon
Wool	Acrylic
Cotton	Rayon
Linen	Spandex
Bamboo	Acetate
Hemp	Polyamide

Polyester and sexual function

Rats fitted with pants made of 100% polyester, a polyester-cotton blend, 100% cotton, or 100% wool were tested for sexual behavior before dressing and again at 6 and 12 months. The polyester and polyester-cotton groups showed a significantly reduced rate of successful mounting compared to controls, worsening from 6 to 12 months, while cotton and wool groups were unaffected. Polyester fabric generated a measurable electrostatic charge on the penis and scrotum; cotton and wool did not. Six months after the pants came off, sexual behavior in all groups returned to baseline. This is an animal study, but the electrostatic mechanism matches the dielectric differences measured directly between cotton/cellulose fibers and polyethylene terephthalate (polyester) fiber above.

Warmth and wool

cold hands and feet are a sign of low metabolism, and cold feet can drive inflammation elsewhere in the body. Wool underwear and thick stockings are a simple first aid for low thyroid symptoms.

Underwear and sperm

Loose underwear goes with better sperm counts in some studies, but not all. In a pilot study, two men switched between tight briefs and loose boxers every three months for a year, and their semen measures fell in tight and rose in loose underwear. Men at a fertility center who mostly wore boxers had 25% higher sperm concentration than other men, with a higher total count and lower FSH. A cohort of 473 men planning a pregnancy found few differences in semen by underwear type, and none that held up after correction, with no effect on time to pregnancy.

Bras

A study of 1,044 women aged 55 to 74 with breast cancer and 469 women without found no link between any aspect of bra wearing and breast cancer.

Underwear and yeast infections

Synthetic underwear goes with more vaginal yeast infections in some studies, though these only show a link and cannot prove cause. Among 470 women in Portugal, those wearing non-cotton underwear had a significantly higher risk of vaginal yeast infection. In a case-control study of 176 young women in Nigeria, tight nylon or synthetic underwear went with nearly 5 times the odds of repeated yeast infections.

Hormones also matter. Ray Peat linked the high rate of yeast infections before periods and in pregnancy to estrogen lowering the IgA antibody on membranes.

Laundry and germs

Washing at 40°C leaves some germs behind. In a lab test, a 40°C wash left bacteria on cotton and polyester and passed some of them to clean fabric in the same load, while a 60°C wash removed them all. A Candida yeast was found in 25% of 99 home washing machines tested in Turkey. In 10 home machines, tumble drying did not significantly lower the germ load passed to test cloths.

Chemicals in fabric

PFAS were found in all 72 stain-resistant children's textiles tested in North America, with the highest levels in school uniforms. Clothing that has taken up plastic softeners from indoor air can raise how much of them passes through the skin.

Flame retardants in children's pajamas and bedding were strong mutagens and a likely cause of cancer.

Synthetic microfibers

In lab-grown human and mouse airway tissue, nylon microfibers held back the growth of new airway lining more than polyester did, and the effect came from substances leaching out of the nylon. See Microplastics.

Red light

Red light and near-infrared light (roughly 600-900 nm) are absorbed by cytochrome oxidase in mitochondria. That step hands electrons to oxygen. Absorption can raise ATP and free the enzyme from inhibitory nitric oxide.

Practical use: red or near-IR lamps on skin and tissue for inflammation, recovery, and local energy. See Mitochondria.

Mechanism

Cytochrome c oxidase is the thyroid-sensitive, oxygen-reacting bottleneck of respiration. Inhibitors include nitric oxide, carbon monoxide, estrogen, PUFA, serotonin, excess iron, radiation, and endotoxin. Red light reactivates the enzyme in the same literature line used for endotoxin and estrogen mitochondrial damage. Red and orange wavelengths penetrate tissue well because water absorbs them weakly, so they can reach cell pigments including cytochrome oxidase and raise ATP. In rats poisoned with methanol, whose breakdown product formic acid blocks cytochrome oxidase, three brief treatments with 670 nm red light partly restored retinal function and kept photoreceptor mitochondria intact. The same light did not change the retina of healthy rats.

Red and orange wavelengths penetrate tissue very effectively, because of their weaker absorption by water, allowing them to react with pigments in the cell, such as cytochrome oxidase, which is activated (or re-activated) by red light, increasing the production of ATP.

— Ray Peat

After UV or other radiation injury, red light can quench ongoing free-radical production in tissue. See Radiation, Inflammation. Pre-treating human skin fibroblasts with non-coherent near-infrared radiation (700-2000 nm), without any measurable temperature rise, protected the cells against subsequent solar UV toxicity; the protection lasted at least 24 hours and did not depend on new cell division or new protein synthesis.

Resting energy expenditure

In a randomized crossover trial in women with obesity, one 12-minute whole-body session of red plus near-infrared light raised resting energy expenditure by about 9% compared with before the session, without changing the mix of fuels burned. In normal-weight women the rise was small. The trial measured no mitochondrial markers, and the rise in energy use did not track the rise in skin temperature. Acute metabolic-rate lift in a stressed metabolic phenotype is not a substitute for thyroid, diet, or PUFA reduction.

Blood glucose

Fifteen minutes of 670 nm photobiomodulation starting 45 min before an oral glucose tolerance test lowered the integrated post-load rise in blood glucose by 27.7% over two hours versus placebo in healthy adults (n=15 per arm), with maximum glucose spiking reduced by 7.5%. Higher mitochondrial glucose demand after red-light stimulation of cytochrome oxidase is the stated mechanism. See Insulin, Mitochondria.

Gut microbiome

Photobiomodulation applied to the abdomen shifted gut microbiome composition in mice, a result the same research group later reported confirming in humans, likely through effects on metabolism and circadian signaling rather than direct contact with gut bacteria.

Fatty liver

In a 12-week randomized trial in 60 obese older adults (65-75) with confirmed non-alcoholic fatty liver disease, adding twice-weekly 660 nm laser (30 min, applied over the abdomen) to a Mediterranean diet outperformed the same diet alone on every measure tracked. Waist circumference fell about 15 cm in the laser group versus about 6-7 cm on diet alone (men: 109.1 to 94.0 cm vs 109.7 to 103.0 cm; women: 99.1 to 84.6 cm vs 100.7 to 93.9 cm); BMI, liver enzymes (AST, ALT, ALP), and the full lipid panel (lower total cholesterol, LDL, triglycerides; higher HDL) all improved more in the laser group, and both groups' baseline values had been matched by randomization. The diet in both arms already accounts for much of the effect, red light added an additional, statistically significant benefit on top of it rather than acting alone. See Fatty liver disease for the PUFA/endotoxin mechanism this fits into.

Cognition and Alzheimer's disease

In a randomized, double-blind, placebo-controlled trial of 93 older adults (age 50 and up) with mild cognitive impairment, 60 days of transcranial photobiomodulation improved cognition on the Montreal Cognitive Assessment more than placebo and raised serum BDNF. The cognitive gain held at a 150-day follow-up. Depression, anxiety, resilience scores, and the neurodegeneration markers NSE and S100B did not differ between groups.

In cell models of Alzheimer's disease (APP/PS1 primary neurons and SH-SY5Y cells expressing human APPswe), photobiomodulation raised cytochrome oxidase activity, ATP, and cAMP, activating PKA and SIRT1. SIRT1 activation increased ADAM10 and lowered BACE1, shifting amyloid precursor protein processing away from the amyloidogenic pathway and reducing amyloid-beta production.

Myopia

In a randomized trial in children with myopia, repeated low-level red light therapy shortened the eye and increased the thickness of the blood-vessel layer beneath the retina, compared with single-vision spectacles. Median axial length shortened by 0.06 mm in the treated group.

Scalp irritation and pruritus

In a randomized, sham-controlled trial in adults with pattern hair loss, daily red LED treatment reduced scalp burning and stinging. Differences in itching and irritation were not statistically significant. The study was funded by the device's maker; the proposed mechanism involved local nitric oxide release, increased ATP production and lower inflammatory cytokines.

Ocular red light and reproductive hormones

In male donkeys during the non-breeding season, 6 hours daily of red LED light directed at a single eye raised plasma testosterone, LH, FSH, AMH, and Activin A while lowering melatonin, restarting reproductive-axis activity without any change to diet or housing; red light outperformed blue and white light in the same trial. This is animal data, not a human trial, but it fits the pattern of light entering through the eye acting on the hypothalamic-pituitary-gonadal axis via melatonin suppression rather than only through direct skin/tissue absorption.

Photoreceptor aging

In adults aged 28-72, brief daily exposure to 670 nm red light improved rod-mediated dark-adapted (night) vision and blue-cone contrast sensitivity specifically in participants over 40. Mitochondrial decline in photoreceptors may drive age-related vision loss. The red (protan) axis improved but not significantly, and the effect was strongest with lower irradiance (8 mW/cm2) rather than higher.

In a retrospective case series, four short sessions of 780 nm laser light over two weeks were followed by better visual acuity in most eyes with age-related macular degeneration.

In women with Hashimoto thyroiditis, 820 nm photobiomodulation (200 mW, 20 s per point at 8 points, twice weekly for 3 weeks) plus vitamin D/selenium improved TSH and thyroid antibodies and reduced weight and waist measures versus supplements alone.

Caveats

Isolated single-wavelength ("pure") red light is not automatically safe just because it is used therapeutically; full-spectrum sources such as incandescent lamps or sunlight do not show the same cancer-growth concern raised for pure red in interview remarks below.

There isn't enough skepticism about what pure red lights are doing, but there is data showing it can make cancer grow, where the incandescent or sunlight type of absorption and resonance, no one has ever seen those effects from a full spectrum.

— Ray Peat

Skin steroid production

Lack of sunlight sits with hypothyroidism, vitamin A deficiency, and protein deficiency when tracing why stress outruns the body's own cortisone. Light on the skin may raise local steroid production without depending on other organs; different steroids may need different frequencies, with orange and red appearing especially important.

Practice (red light)

Distance and time set dose. Skin should warm without burning. Eyes need care with bright LEDs. Morning and post-injury use appear in practical bioenergetic notes. Pair with whole-body supports: thyroid, sugar oxidation, lower PUFA. See Thyroid, CO2, PUFA, Activities.

See also

• Aspirin
• Red light
• Circadian rhythm

Quotes

I suspect that light on the skin directly increases the skin's production of steroids, without depending on other organs. Different steroids probably involve different frequencies of light, but orange and red light seem to be important frequencies.

— Ray Peat

Although direct sunlight damages skin, people generally are healthier in proportion to their exposure to sunlight. Ultraviolet and blue light produce free radicals in living material, but they penetrate only a few millimeters into tissue. Yellow, orange, red, and infrared rays penetrate much more deeply.

— Ray Peat

It turns out that daylight stimulates our ability to use oxygen for energy production, and protects our tissues from some of the free-radical toxins that are produced by normal metabolism, by stress, or by radiation.

— Ray Peat

If artificial light is bright enough, it is as effective as sunlight at stopping the stress reaction, but people seldom use lights that are bright enough. Generally, people and animals are healthier when days are longer than 12 hours, that is, after March 21 and before September 20.

— Ray Peat

So my present thought is that darkness does in some way "poison" respiration, and that light enhances it, possibly by the indirect mechanism of disposing of electrons that have leaked out of the usable-energy pathway.

— Ray Peat

The light-induced hormonal changes which increase fertility also increase general vitality and decrease the problems associated with aging. The hormonal changes occuring in winter and at night aggravate degenerative processes and increase mortality in all of the species that have been studied, including humans, in spite of the use of artificial lights.

— Ray Peat