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 (HR 0.77, 95% CI 0.63-0.95); occasional users had a 14% lower hazard (HR 0.86, 95% CI 0.77-0.96).

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.
• Sunlight exposure is associated with lower influenza incidence.
• 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.
• Direct sunlight is required for dopamine synthesis.

Mortality

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."

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

Richard Weller, a dermatologist at the University of Edinburgh and co-author of the 2026 UK Biobank glucose study and the 2024 UK Biobank mortality study above, has separately argued there is no good evidence that sun exposure shortens life, and points to a Danish cohort in which people already diagnosed with skin cancer had lower subsequent rates of heart disease and death than the general population.

Skin cancer risk in context

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.

A meta-analysis of 29 studies (313,717 participants, 10,670 cases) found no significant overall association between sunscreen use and skin cancer (odds ratio 1.08) or melanoma specifically (odds ratio 1.10); a real protective association seen in pre-1980s studies (odds ratio 2.35) weakened steadily and disappeared from the early 1990s onward, a shift the study authors attribute to changed sunscreen formulations and study methods rather than sunscreen itself becoming protective.

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.

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.
• 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.

See also

• Light
• Vitamin D3
• Diabetes
• Glucose
• Red light