No credible evidence supports the idea that exposing your testicles to sunlight improves testosterone levels, fertility, or sexual health. The claim gained traction in online wellness circles partly thanks to a single 1939 study of men with depression, which reported that UV light directed at the genitals increased a testosterone metabolite in urine by “nearly 200%.” That study had a tiny sample, no control group, and methods that would not pass modern scrutiny. Meanwhile, the actual science on sunlight, vitamin D, and male reproductive function tells a more complicated and less exciting story, and it comes with warnings about heat damage and skin cancer risk that the viral clips tend to leave out.
Where the Idea Comes From
A 2023 observational study in JMIR Dermatology tracked public interest in “testicle tanning” and traced the claim’s origins. Proponents frequently cite the 1939 study mentioned above, which exposed a small group of men with depressive mental illness to UV radiation on their genitals and measured urinary androsterone, a breakdown product of testosterone. Beyond the obvious design problems, the study did not measure serum testosterone directly, the population was not healthy, and no placebo group existed for comparison. The same review noted that while some research has linked UV exposure to changes in sex steroid hormones, those studies either did not involve human participants or did not evaluate UV radiation directed specifically at the genitals.1PubMed Central. Evaluating the Public’s Interest in Testicle Tanning: Observational Study
The most serious modern study that proponents could point to is a 2021 paper in Cell Reports, which found that UVB light exposure on the skin of mice triggered a hormonal cascade involving the hypothalamic-pituitary-gonadal axis, resulting in elevated sex steroid hormones and changes in sexual behavior. In humans, the researchers found correlations between reported sun exposure and both testosterone levels and self-reported romantic passion. But the human portion relied on questionnaires and correlational data, and the UVB exposure in the mouse experiments was applied to shaved back skin, not to the genitals. The mechanism the researchers proposed runs through the skin as a sensory organ sending signals to the brain, which then signals the gonads. If that pathway holds up in humans, it would mean general skin sun exposure matters, not genital-specific exposure.2PubMed Central. Skin exposure to UVB light induces a skin-brain-gonad axis and sexual behavior
Vitamin D and Testosterone Are Connected, But Not the Way You Think
There is a real biological relationship between vitamin D and male reproductive function. The vitamin D receptor and vitamin D metabolizing enzymes are present in the testes, in mature sperm, and in the ejaculatory tract, suggesting that vitamin D plays some role in regulating male fertility and hormone production.3Oxford Academic. Vitamin D metabolism, sex hormones, and male reproductive function Several observational studies have found that men with higher vitamin D levels tend to have somewhat higher testosterone. A large European study found that vitamin D was positively associated with both total and free testosterone, though the relationship plateaued once vitamin D levels reached a sufficient range, roughly above 75 to 85 nmol/L.4PubMed Central. Association between plasma 25-OH vitamin D and testosterone levels in men An Austrian study similarly reported that men with sufficient vitamin D levels had higher testosterone than those who were deficient, and that both vitamin D and testosterone followed the same seasonal curve, dipping in late winter and peaking in late summer.5PubMed. Association of vitamin D status with serum androgen levels in men
A 2023 systematic review looking at the broader literature found converging support for a link. A Mendelian randomization analysis in over 4,200 Chinese men suggested that genetically lower vitamin D levels were associated with lower testosterone, hinting at a possible causal role. A cross-sectional study of Dutch men found that those with vitamin D levels below 25 nmol/L had lower total and bioavailable testosterone than men with higher levels.6PubMed Central. Association Between Vitamin D Deficiency and Testosterone Levels in Adult Males: A Systematic Review
Here is the catch: these are all observational findings. Men who spend more time outdoors, exercise more, and maintain a healthy weight also tend to have both higher vitamin D and higher testosterone, so disentangling cause from correlation is tricky. And the pathway from sunlight to vitamin D to testosterone runs through skin all over the body, not the scrotal skin specifically. Your forearms, face, and legs produce vitamin D from sun exposure just as well.
What Happens When You Actually Give People Vitamin D Supplements
The interventional evidence is mixed, and that matters because it is the closest thing we have to a controlled test. One frequently cited trial gave vitamin D supplements to overweight men who were deficient at baseline. Over the course of a year, the supplemented group saw total testosterone rise from about 10.7 nmol/L to about 13.4 nmol/L, with similar increases in bioactive and free testosterone. The placebo group saw no change.7PubMed. Effect of vitamin D supplementation on testosterone levels in men That sounds promising, but there is a crucial detail: these were men who started out deficient in vitamin D and had testosterone at the lower end of the normal range. Correcting a deficiency is not the same as boosting levels above normal.
A larger, more rigorous randomized controlled trial tested the same idea in healthy men who were not deficient. These participants had a mean vitamin D level of about 53 nmol/L at baseline and normal testosterone. After supplementation, no significant effect on testosterone was found.8The Journal of Clinical Endocrinology & Metabolism. Vitamin D and Testosterone in Healthy Men: A Randomized Controlled Trial The takeaway from these two studies together is straightforward: if you are genuinely vitamin D deficient, correcting that deficiency may modestly improve testosterone. If your vitamin D is already in a healthy range, adding more does not appear to push testosterone higher. And none of this requires exposing your genitals to the sun; an ordinary supplement or regular outdoor activity does the same thing.
The Heat Problem Is Real
There is a reason the testicles hang outside the body. Spermatogenesis, the process of producing sperm, requires a temperature a few degrees below core body temperature. The scrotal sac, the cremaster muscle that raises and lowers the testes, and a network of blood vessels called the pampiniform plexus all work together to keep testicular temperature in a narrow range.9PubMed Central. Presence of Arteriovenous Communication between Left Testicular Vessels and Its Clinical Significance When that cooling system is overwhelmed, things go wrong quickly.
Elevated scrotal temperature triggers oxidative stress in germ cells, damages sperm DNA, produces morphologically abnormal sperm, and causes excessive cell death among developing sperm cells. This is not theoretical: studies in both animal models and human clinical populations consistently show that heat stress impairs sperm quality. Everyday factors like wearing tight clothing, sitting for long periods, or simply living in a hotter climate can measurably raise testicular temperature enough to cause problems.10PubMed. The effects and molecular mechanism of heat stress on spermatogenesis and the mitigation measures11PubMed. Causes, effects and molecular mechanisms of testicular heat stress
Sitting or lying in direct sunlight with exposed genitals adds radiant heat on top of ambient heat. Even moderate scrotal heat stress in animal studies, a single 30-minute exposure at 42°C in mice, compromised sperm DNA integrity enough to raise concerns about downstream effects on fertilization and embryo development.12Reproduction. Impact of a mild scrotal heat stress on DNA integrity in murine spermatozoa A ram study found that heat stress increased sperm DNA fragmentation without altering protamination patterns, indicating that the damage was oxidative rather than structural, but no less concerning for fertility.13PubMed Central. Effect of Heat Stress on Sperm DNA: Protamine Assessment in Ram Spermatozoa and Testicle In other words, the very act of sunbathing your testicles could undermine sperm quality through the same thermal mechanism that makes hot tubs and laptop use a concern for men trying to conceive.
Vitamin D and Sperm Quality
Separate from the testosterone question, researchers have explored whether vitamin D affects sperm health. A review in the World Journal of Men’s Health concluded that most studies point toward a positive effect of vitamin D on male fertility potential, particularly through improved sperm motility.14PubMed Central. Vitamin D and Male Fertility: An Updated Review But when this was tested in a rigorous randomized trial involving infertile couples, the results were less encouraging. Men receiving vitamin D supplementation showed no improvement in total sperm count or sperm concentration compared to the placebo group, and the groups did not differ in motility at the end of treatment either.15The Journal of Clinical Endocrinology & Metabolism. Effects of Vitamin D Supplementation on Semen Quality, Reproductive Hormones, and Live Birth Rate: A Randomized Clinical Trial
The pattern mirrors the testosterone findings: observational data looks favorable, but controlled interventions have not consistently delivered. Vitamin D is probably important for maintaining normal reproductive function when you are deficient. It does not appear to be a performance enhancer once your levels are adequate.
Seasonal Testosterone Swings Are Smaller Than You Would Expect
If sunlight drives testosterone, you would expect clear seasonal patterns: high testosterone in summer, low in winter. Some studies do find this. The Austrian study mentioned earlier reported a seasonal nadir for testosterone around March and a peak around August, roughly tracking vitamin D levels.5PubMed. Association of vitamin D status with serum androgen levels in men A Danish study following men over 17 months found seasonal variation in both LH and testosterone, but concluded that the variation in testosterone could be explained by variation in LH, and that LH levels seemed linked to air temperature rather than to daylight hours or sunshine.16The Journal of Clinical Endocrinology & Metabolism. Variation in Levels of Serum Inhibin B, Testosterone, Estradiol, Luteinizing Hormone, Follicle-Stimulating Hormone, and Sex Hormone-Binding Globulin in Monthly Samples from Healthy Men during a 17-Month Period: Possible Effects of Seasons
Other studies have failed to find meaningful seasonal variation at all. A large analysis from two institutional databases, one in Pittsburgh and one in Miami, found no statistically significant seasonal variation in testosterone levels for the overall cohort or for any age group.17PubMed Central. Seasonal Variation in Serum Testosterone Levels: Evidence from 2 Large Institutional Databases A study of over 8,300 middle-aged men actually found a negative relationship between testosterone and both daylight duration and outdoor temperature, the opposite of what the “more sun equals more testosterone” narrative would predict.18PubMed. Monthly Variations in Serum Testosterone Levels: Results from Testosterone Screening of 8,367 Middle-Aged Men
The seasonal picture is genuinely messy. Temperature, exercise patterns, sleep, diet, and body composition all change with the seasons, making it nearly impossible to isolate sunlight as the driver. Whatever seasonal testosterone variation exists in some populations, it is modest and clearly not as simple as “more sun, more testosterone.”
Red Light Therapy Is Not the Same as Sunlight
Some online sources conflate testicle sunning with red light therapy, but these are completely different interventions. Low-level laser therapy, sometimes called photobiomodulation, uses specific wavelengths of red or near-infrared light, in the 600 to 1,000 nanometer range, at controlled power levels. The proposed mechanism involves activating an enzyme in mitochondria called cytochrome C oxidase, which could increase cellular energy production and reduce oxidative stress.19PubMed Central. Low-level laser therapy for the treatment of male infertility and erectile dysfunction Research into this for male fertility and erectile dysfunction is still early-stage, and most of the evidence comes from animal models or small clinical trials. Even if red light therapy eventually proves useful for male reproductive health, it would not validate the idea of sitting in the sun with your pants down. Sunlight contains a broad spectrum of radiation including ultraviolet wavelengths that damage skin, and delivers heat that can harm sperm. A controlled LED device delivering a narrow band of red light to a specific area is a fundamentally different exposure.
Light, the Brain, and Sexual Behavior
There is one more piece of the puzzle that is genuinely interesting but often misunderstood. Light exposure affects the brain in ways that influence reproductive physiology, primarily through the eyes and the hypothalamus rather than through the skin. Melatonin, which the pineal gland produces in darkness, acts as a local modulator of hormone production in the Leydig cells that manufacture testosterone and influences energy metabolism in the Sertoli cells that support sperm development. Bright daytime light suppresses melatonin, and this suppression may indirectly support normal testosterone rhythms.
An animal study in diurnal rodents found that males housed in dim daylight conditions showed disrupted sexual behavior, mounting females less frequently, even though their testosterone levels and testes size were unchanged compared to males in bright daylight. The effect appeared to operate through changes in brain receptor expression rather than through hormones directly.20Frontiers in Behavioral Neuroscience. Low Daytime Light Intensity Disrupts Male Copulatory Behavior, and Upregulates Medial Preoptic Area Steroid Hormone and Dopamine Receptor Expression, in a Diurnal Rodent Model of Seasonal Affective Disorder This matters because it suggests that sufficient bright light during the day supports reproductive behavior through brain pathways, not through direct genital exposure. Getting outside during the day, keeping your indoor environment well-lit, and maintaining normal circadian rhythms are all supported by evidence. Pointing sunlight at your testicles is not.
Skin Sensitivity and Cancer Risk in the Genital Area
Scrotal skin is thinner than skin on most other body parts and has different absorption characteristics. Research on percutaneous absorption identifies the application site as one of the most important factors determining how much of a substance or radiation penetrates the skin. Genital skin is among the most permeable regions, and factors like hydration, temperature, and skin integrity further modify absorption.21SpringerLink (Am J Clin Dermatol). Twenty Clinically Pertinent Factors/Observations for Percutaneous Absorption in Humans UV radiation damage to this delicate tissue is a real concern. The JMIR Dermatology study on testicle tanning noted that excessive UV exposure to the genital area may lead to higher rates of genital tumor formation.1PubMed Central. Evaluating the Public’s Interest in Testicle Tanning: Observational Study Penile and scrotal skin cancers are uncommon, but they do occur, and deliberately exposing an area that rarely sees UV light to direct sun is an effective way to increase risk. Skin that has not built up any UV tolerance is especially vulnerable to sunburn, and a sunburn in the genital area is as unpleasant as it sounds.
What Actually Supports Male Reproductive Health
If the underlying concern is low testosterone or poor sperm quality, the evidence points toward interventions that are less dramatic than genital sun exposure but far better supported. Getting adequate vitamin D matters, and if you are deficient, supplementation or regular whole-body sun exposure in moderation can correct that. Maintaining a healthy weight has one of the strongest associations with testosterone levels in the research literature. Regular exercise, adequate sleep, and managing chronic stress all support the hormonal axis. Avoiding prolonged heat exposure to the groin area, whether from hot baths, tight clothing, or laptops resting on your lap, protects sperm quality. If you suspect you have a genuine hormonal deficiency, a blood test from your doctor is the appropriate starting point, not an internet protocol involving UV exposure to sensitive anatomy.
The appeal of testicle sunning probably lies in its simplicity and its transgressive novelty: a free, natural, slightly outrageous “hack.” But the science it is built on is either preliminary, misinterpreted, or flatly contradicted by more rigorous research. The one intervention in this space that has actually shown a testosterone effect in a controlled trial, vitamin D supplementation for deficient men, does not require removing any clothing at all.