The intellectual foundation of the argument comes from a 2020 commentary by physiologist George Billman, titled “Homeostasis: The Underappreciated and Far Too Often Ignored Central Organizing Principle of Physiology.” Billman defined homeostasis as a self-regulating process by which an organism maintains internal stability while adjusting to changing external conditions, and described it as a dynamic process that can alter internal conditions as required to survive external challenges. Chiou’s review of the literature suggests that few if any studies have applied this principle to the effects of non-burning sunlight exposure on human skin aging. In the article, “non-burning” is used descriptively, meaning regular daily outdoor activities that do not produce burning or erythema, the visible reddening of skin. The author explicitly notes that the term is not intended to imply the absence of molecular photodamage or to define a validated safe exposure threshold.

The biological reasoning behind the hypothesis starts with a striking estimate: the DNA in each human cell may sustain tens of thousands of lesions per day. According to the homeostasis principle, in normal healthy subjects these large numbers of lesions may be largely repaired or neutralized by the body’s highly efficient repair and defense mechanisms. Chiou extends this logic to sunlight, postulating that since birth the human body must have already developed extremely effective mechanisms to counter or neutralize damage caused by non-burning sunlight. The author draws an analogy to a mild fever caused by a mild infection, which may resolve within days without any special treatment. On this view, daily non-burning sun exposure may not significantly accelerate skin aging through degradation of collagen and superficial capillary tissue, two potentially important manifestations of premature aging or photoaging. Importantly, the hypothesis does not extend to other aging symptoms such as age spots, freckles, melasma, and elastosis, which the author sets aside as outside its scope.

The empirical support comes from two clinical studies whose data, when plotted on a semilogarithmic scale, show approximately exponential age-related declines. In the collagen study, skin samples were taken from the sun-exposed extensor surface of the forearm in 74 male subjects aged 9 to 90 years. The approximate linear semilogarithmic plot, with a coefficient of determination of about 0.90, indicates no apparent age-dependent acceleration in collagen loss; if such acceleration existed, the slope would become progressively steeper with time. The mean half-life of collagen content in these samples was calculated to be 44 years. Because the samples came from a habitually sun-exposed site, the author interprets the smooth, unaccelerated decline as consistent with the idea that ordinary sun exposure did not measurably hasten collagen degradation in these subjects.

The second dataset concerns superficial capillary density, a measure of the small blood vessels near the skin surface. Samples were taken from both sun-exposed sites, the forehead and the back of the hand, and a sun-protected site, the volar forearm, in 50 subjects aged 20 to 74, with the data plotted in groups of 10 subjects. This semilogarithmic plot showed an even tighter fit, with a coefficient of determination of about 0.95, and the mean half-life of capillary density was 29 years. Crucially, similar exponential aging patterns were observed in sun-exposed and sun-protected skin alike. If chronic everyday sunlight were markedly accelerating the loss of these superficial capillaries, one might expect the decline in exposed sites to outpace that in protected sites, which the plotted data do not appear to show.

These findings sit uneasily beside the conventional concept of photoaging, which holds that chronic exposure to sunlight in daily life is expected to result in premature skin aging. Chiou attempts to reconcile the difference by pointing to a reported study in which chronic sun exposure caused histological features of chronic skin inflammation without detectable clinical or molecular abnormalities in eight subjects aged 47 to 79, when sun-exposed preauricular skin was compared with sun-protected postauricular skin. The author suggests this study may provide some support for the hypothesis regarding collagen degradation, and raises the possibility that substantial collagen degradation may require more intense sunlight exposure, such as exposure sufficient to cause erythema or burns. The article also stresses the importance of radiation intensity when studying sunlight’s effects on skin, arguing that findings obtained at very high intensities may not translate to ordinary outdoor conditions.

That point about intensity is illustrated by a laboratory study the author describes as excellent, in which irradiation of human buttock skin with various fractions of minimum erythema doses of UVB rapidly induced extracellular matrix metalloproteinases, enzymes that might degrade collagen and result in premature skin aging such as sagging and wrinkling upon repeated chronic sun exposure. The radiation intensity used in that experiment was 0.5 milliwatts per square centimeter, which Chiou estimates to be equivalent to approximately 20 times the intensity corresponding to a UV Index of 10. This raises the question of whether such a high radiation intensity might produce effects that differ from those occurring during real-world exposure to non-burning sunlight. In other words, the author suggests that some laboratory evidence for photoaging may have been generated under conditions far more intense than the daily exposures the hypothesis addresses.

The perspective also ventures into less familiar territory by highlighting a potential contributor to intrinsic skin aging that the author believes has been largely overlooked: the heart. Chiou argues that because the heart delivers vital nutrients, including water, to the skin to maintain its function and structural integrity, age-related changes in cardiac function may be a potential contributor to intrinsic skin aging. The article notes that dehydration of the epithelium from UV radiation, which may affect skin barrier function and induce skin pathogenesis, has received attention in recent years. Supporting this line of thought, the aging of normal human hearts, expressed in terms of cardiac index or cardiac output per square meter of body surface area, was also found to follow an approximate exponential decay, with a mean half-life of 82 years in 77 subjects aged 12 to 82. The author describes the heart as the body’s engine and suggests its aging deserves consideration as an independent factor in how skin ages.

The article is explicit that its hypothesis does not claim non-burning sunlight causes no damage to human skin. Rather, it proposes that the body’s repair and defense mechanisms may be sufficiently effective to prevent such damage from causing significant acceleration of the two major signs of skin aging under discussion, namely degradation of collagen and reduction of superficial capillary density. The author also emphasizes several important limitations. The available data are cross-sectional and were not designed to quantify individual cumulative sunlight exposure. In the collagen study, sun-exposed skin was not directly compared with matched sun-protected skin, while the capillary-density study compared different anatomical sites that may differ for reasons other than sunlight exposure. In addition, the intensity, duration, and lifetime pattern of sunlight exposure were not characterized. Accordingly, the observations should be regarded as hypothesis-generating and cannot establish that non-burning sun exposure does not accelerate skin aging.

Looking forward, the author proposes that appropriately designed prospective studies could test the hypothesis directly. Future research comparing sun-exposed and sun-protected skin over a wide range of ages and skin phenotypes, across sunlight intensities such as UV Index 3, 6, and 9, and over defined time periods such as 20 and 60 minutes, may help to explore the applicability and limitations of the hypothesis in diverse populations and settings. Until such studies are carried out, the proposal remains an open challenge to a long-standing concept in skin biology, offered in the hope of stimulating further discussion and research rather than as a settled verdict on the sun’s role in how our skin ages.

Subject of Research: A hypothesis applying the homeostasis principle to the effects of non-burning sun exposure on human skin aging

Article Title: The homeostasis principle for predicting the effect of non-burning sun exposure on human skin aging

Article References: Chiou, W. L. (2026). The homeostasis principle for predicting the effect of non-burning sun exposure on human skin aging. Frontiers in Photobiology, 4, Article 1979864. https://doi.org/10.3389/fphbi.2026.1979864

Image Credits: AI Generated

DOI: 10.3389/fphbi.2026.1979864

Keywords: skin aging, photoaging, homeostasis, collagen, capillary density, non-burning sun exposure, UV index, radiation intensity, cardiac aging, DNA repair, Frontiers in Photobiology, perspective article