Sunlight

Sunlight exposure lowers acute blood glucose and reduces long-term type 2 diabetes risk, alongside its established role in vitamin D synthesis, dopamine production, and seasonal mood. Mainstream dermatology treats sun exposure as a pure liability because of skin cancer risk, but cohort studies measuring all-cause mortality find the opposite: avoiding the sun tracks a mortality burden comparable to smoking.

Sunlight can ...cure depression & increase intelligence.

— Ray Peat

Light promotes glucose oxidation, and is known to activate the key respiratory enzyme. Winter sickness (including lethargy and weight gain), and night stress, have to be included within the idea of the "respiratory defect," shifting to the antirespiratory production of lactic acid, and damaging the mitochondria.

— Ray Peat

Blood glucose

In a UK Biobank cohort study using satellite ultraviolet-A (UVA) data matched to each participant's clinic date and residence, a 1 W/m² increase in the 7-day average sunlight exposure before a blood test was associated with 0.027 mmol/L lower glucose in winter and 0.007 mmol/L lower glucose in spring, roughly four times the winter effect versus spring. The 2-day average showed a smaller but still winter-strongest pattern (0.015 mmol/L lower per 1 W/m² in winter). The dose-response was non-linear: the steepest drops in glucose occurred at low sunlight levels, then plateaued as exposure rose further, meaning a person's first exposure after a dark stretch matters more than adding sun on top of an already-sunny week. Frequent solarium users had a 23% lower hazard of type 2 diabetes over follow-up than non-users. Occasional users had a 14% lower hazard.

Higher levels of ambient UVB across 186 countries correlated with lower colorectal cancer incidence at every age group studied, an effect the authors attribute to UVB-driven vitamin D synthesis.

Other benefits

• Improves oxygen use for energy and protects against free radicals from metabolism, stress, or radiation.
• Reduces depression, boosts immunity, and stimulates metabolism, reducing food cravings.
• Suppresses melatonin, the darkness-driven hormone linked to brain sluggishness, reduced fertility and immunity, and organ shrinkage; death rates run higher in winter and at night.
• Offsets seasonal light deficiency behind winter illnesses.
• Light on the skin may raise local steroid production without depending on other organs; orange and red frequencies appear especially important.

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

• Sunlight exposure is associated with lower influenza incidence.
• In a matched case-control study in Melbourne, children who spent more leisure time in the sun the previous summer or winter were less likely to have inflammatory bowel disease, with risk falling steadily as sun time rose.
• In infants of mothers with perinatal depression, whose early motor development lagged and whose cortisol ran high, 7–14 hours of sunlight a week plus a standard 400 IU vitamin D supplement gave the best motor scores and the lowest hair cortisol, beating vitamin D supplements alone.
• Physical activity done outdoors improves cognition more than the identical activity done indoors. In a trial of 45 children aged 11-13, outdoor exercise sessions produced faster reaction times and higher accuracy on working-memory and attention tasks (Stroop, Sternberg, Flanker) than indoor sessions, an effect still present 45 minutes after the activity ended.
• In rats, light raises dopamine synthesis in retinal neurons. Healthy adults with high sunshine exposure had more striatal dopamine D2/D3 receptor availability than those with low exposure.

Mortality

Although exposure to sun does contribute to aging of the skin, people who spend years working outdoors have a reduced incidence of cancer of internal organs.

— Ray Peat

Twenty years of follow-up on nearly 30,000 Swedish women (the Melanoma in Southern Sweden cohort) found all-cause mortality roughly doubled in women who avoided sun exposure compared to those with the highest sun and tanning-bed exposure, an effect that held after adjusting for smoking, BMI, income, and education; sun avoidance shortened life expectancy by 0.6 to 2.1 years and death from cardiovascular disease accounted for most of the difference. A later competing-risk analysis of the same cohort found sun avoidance a risk factor for death "of a similar magnitude as smoking." The data are observational, and the excess deaths tied to sun avoidance were a small share of all deaths in the cohort.

A meta-analysis of observational cohorts (849,412 participants) attributed 12.8% of all US deaths (about 340,000 per year) and 9.4% of European deaths (about 480,000 per year) to serum 25(OH)D below 75 nmol/L, with inverse associations for cardiovascular, cancer, and other-cause mortality. An expert review framed those figures as evidence that insufficient sun exposure is a public health problem, and listed higher incidence of breast and colorectal cancer, hypertension, cardiovascular disease, metabolic syndrome, multiple sclerosis, Alzheimer's disease, autism, asthma, type 1 diabetes, and myopia among linked outcomes. Oral vitamin D supplementation has not been convincingly shown to prevent that disease set, so serum 25(OH)D may mark beneficial sun exposure rather than mediate it solely through vitamin D; candidate non-vitamin-D mechanisms include nitric oxide release from skin and direct ultraviolet effects on circulating blood cells.

A larger 2024 UK Biobank analysis (N=453,026) used two objective UV exposure measures, solarium use and residential shortwave radiation, both validated against measured vitamin D levels, and found both inversely associated with all-cause, cardiovascular, and cancer mortality; solarium users also had lower non-cardiovascular, non-cancer mortality. Neither exposure raised melanoma mortality, with a non-significant trend toward lower melanoma mortality among solarium users.

In fact, the evidence accumulated so far suggests that avoiding the sun has the same cumulative risks for health as picking up a pack-a-day smoking habit.

— Haidut

There is no good evidence that sun exposure shortens life. In a Danish cohort, people already diagnosed with skin cancer had lower subsequent rates of heart disease and death than the general population.

UVB, vitamin D, and cancer ecology

A review of ecological studies of geographic cancer incidence and/or mortality reported strong inverse correlations with solar UVB for 15 cancer types (bladder, breast, cervical, colon, endometrial, esophageal, gastric, lung, ovarian, pancreatic, rectal, renal, and vulvar cancer, plus Hodgkin and non-Hodgkin lymphoma), with weaker evidence for several others; the authors framed the pattern under the UVB–vitamin D–cancer hypothesis.

Skin cancer risk in context

If you avoid sunlight or put on sunscreens because you are afraid of getting skin cancer, the avoidance of sun is very carcinogenic.

— Ray Peat

Roughly 99% of skin cancers are non-melanoma basal cell or squamous cell carcinomas, which are almost never fatal; melanoma, the deadly form, most often appears on skin that rarely sees the sun, is more closely tied to estrogen and pollutant exposure than to sun exposure, and has not been consistently linked to sun exposure. Skin aging from sun exposure is influenced by diet: PUFA accelerates wrinkling, while saturated fats such as coconut oil protect against it. In hairless mice, UV skin cancer and photoimmunosuppression intensified as dietary polyunsaturated sunflower oil rose within a fixed 20% fat diet; 20% saturated fat nearly abolished acute UV suppression of contact hypersensitivity.

Fluorescent lighting at work was proposed in 1982 as one indoor contributor to melanoma, after an Australian case-control study found about double the risk in exposed women; most later studies did not confirm it, and the details are on Light.

A meta-analysis of mostly case-control studies found no link between sunscreen use and skin cancer, including melanoma. In the studies published before the 1980s, sunscreen users had a clearly higher melanoma risk; that link weakened steadily and was no longer significant from the early 1990s onward, and the pooled evidence did not show the expected protection either.

Basal cell carcinoma can be managed with excision or topical aspirin near the lesion. Dietary steps that limit PUFA peroxidation in the skin, higher saturated fat intake, aspirin, niacinamide, vitamin E, and progesterone, are described as reducing skin damage from sun exposure without avoiding the sun itself.

Sunshine is free nutrition that must be utilized at every given opportunity. Every day the sun shines you must consume its light. Better than the most expensive D3 supplement, UV lamp, and infrared panel.

— Ray Peat

Chemical sunscreens

Common chemical UV-filter ingredients (oxybenzone, avobenzone, octocrylene, ecamsule) absorb into the bloodstream at levels exceeding the FDA's own safety threshold after just one day of use, with blood concentrations still rising after four days of continued application. Oxybenzone specifically is associated with lower total testosterone in adolescent boys in NHANES data. Chemical sunscreens (PABA and its derivatives were an earlier example) are best avoided in favor of reflective mineral ones, zinc oxide or titanium dioxide, when sun protection beyond moderation is needed.

Zinc oxide

Zinc oxide least harmful sunscreen. Yes.

— Ray Peat

Zinc oxide works as a physical barrier instead of a hormone-active chemical filter. A stable-isotope tracing study found only small, trace amounts of zinc absorbed through skin after twice-daily sunscreen application over five days, far below the systemic exposure levels seen with chemical filters.

• Badger Sport Mineral Sunscreen SPF 40: non-nano zinc oxide only, no chemical filters, 4-ingredient formula.
• Antioxidants such as vitamin E, aspirin, coconut oil, and carotene mitigate UV damage when ingested or applied.

Vitamin D synthesis

Sun exposure on bare skin is the main natural route to cholecalciferol (vitamin D3), the starting form later converted in the liver and kidneys. See Vitamin D3 for dosing, winter supplementation, and the hormonal mechanism.

Skin production of the vitamin D3 precursor limits itself. Under prolonged simulated sunlight, the precursor in human skin plateaued at about 10 to 15% of the original 7-DHC (its cholesterol-derived substrate), and further exposure converted it into lumisterol and a second biologically inert isomer. Darker skin and higher latitude lengthened the exposure needed to reach the plateau without lowering the maximum.

Sun and a vitamin D pill had opposite effects on blood lipids in a 6-month open-label randomized trial in vitamin D-deficient Indian men aged 40 to 60 (25(OH)D below 50 nmol/L). At least 20 extra minutes of sun on face and forearms between 11 a.m. and 3 p.m. (50 randomized, 32 completed) and oral cholecalciferol 1000 IU/day (50 randomized, 37 completed) both raised serum 25(OH)D, with a larger rise on the pill. Total, LDL and HDL cholesterol fell significantly in the sun group; the pill group showed significant rises in total and HDL cholesterol and a non-significant rise in LDL.

Practical advice

• Avoid overexposure to prevent immunosuppression.
• Reduce unsaturated fats in the diet; use coconut oil on skin and in food for protection.
• In fall and winter, use very bright incandescent lights daily from sunset until bedtime.
• Expose as much skin as possible to bright light; even a minute is better than nothing. Thin, light-colored clothing still transmits a considerable amount of light. Infrared bulbs with clear glass are especially beneficial; special low-temperature red lights are also available.
• Outside the tropics, obtain sun exposure incrementally during high-UV seasons at not more than 5 to 30 minutes a day (depending on skin type and UV index), in season-appropriate clothing, avoiding sunburn; close the eyes or use sunglasses that filter UV.
• Sunlight through a window carries less ultraviolet light than direct sun exposure; plastic film containing copper can also cut the immunosuppressive UV component while letting visible light through.
• Avoid sun-blocking lotions other than the simply reflective type, zinc oxide or titanium dioxide.
• Vitamin E and aspirin reduce the harmful effects of sunburn even when applied topically after sun exposure. Vitamin E often contains some soy oil, so small doses of about 100 mg per day are recommended.

Grounding

Grounding, also called earthing, means direct skin contact with the earth or a wired connection to an electrical ground, such as walking barefoot or sleeping on a grounded sheet. The human research is small, and most of it comes from authors tied to companies that sell grounding products.

Peat's view

Ray Peat linked grounding to the electrical field that living skin projects, a field that depends on respiration.

They talk about it as a sort of a buffer to the electrons that are being produced metabolically.

— Ray Peat

The skin field

Skin kept in a culture dish goes on projecting a field until cyanide stops its respiration, and the field returns once the cyanide is washed out. On this account the charge is made by metabolism, not gathered passively by friction.

Barefoot and grounded beds

Peat thought daytime barefoot contact with the ground was enough by itself, with grounded bedding as an option for homes with strong 50 or 60 cycle fields. The same idea appears on the Clothes page, where synthetic fabrics and rubber soles keep the body insulated.

Negative ions

Breathing negatively ionized air speeds the breakdown of serotonin in the lungs, according to a Peat newsletter.

Human studies

Cortisol and sleep

In a pilot study, 12 people with poor sleep slept on a conductive mattress pad for eight weeks, and their night-time cortisol fell. They also reported better sleep and less pain, but nobody in the study was blinded.

Blood

Two hours of grounding through patches on the hands and feet made red blood cells clump less in ten healthy adults. The title's claim about blood viscosity was not measured directly.

Thyroid and minerals

A Polish group found that grounding during night rest lowered free T3 and raised free T4 and TSH. Read at face value, lower T3 with higher TSH points toward lower thyroid output, which does not fit the claims made for grounding.

Pain and mood

In a small randomized trial, 16 massage therapists reported less pain, fatigue and low mood during the weeks they were grounded.

Evidence quality

Every human study above is small, short and mostly unblinded, and several come from authors with a stake in grounding products. No large independent trial has tested grounding against a sham connection.

See also

• Cortisol
• Sleep
• Inflammation
• Radiation
• Red light