
Interview With Dr. Alexander Wunsch On International Day of Light
Light is one of the most important, yet misunderstood, drivers of your health. Sunlight provides a full spectrum of wavelengths, which humans have co-evolved with to orchestrate vital biological functions. Yet, this relationship is not without complexity.
To explore these intricacies, we sat down with Dr. Alexander Wunsch, MD, PhD—a leading authority in photobiology and light medicine. A German physician, researcher, and author, Dr. Wunsch has spent decades investigating how light interacts with human biology and serves as the Chief Science Officer at Ra Optics. Our conversation took place in honor of the International Day of Light, diving into the complexities of photobiomodulation, skin phototypes, vitamin D levels, and the often misunderstood relationship between sun exposure and health.
Ra Optics: Regarding red light therapy, some research indicates that it works best at specific times of day, notably in the morning. Is there an ideal time to use photobiomodulation, or is it beneficial throughout the day?
Dr. Alexander Wunsch
All our bodily functions are dependent on circadian rhythms. For instance, studies by Professor Glen Jeffery on improved color contrast in the retina have shown that the beneficial effects only occur when treatment is administered during the morning hours. Despite this, chronobiologically-adjusted treatments are generally absent in the professional photobiomodulation field. The World Association for Laser Therapy (WALT), for example, does not take into account chronobiological parameters in its treatment recommendations.
Typically, one wouldn’t expect regeneration to occur during the day—it’s more associated with nighttime activity during sleep. However, this likely depends on the specific tissue being targeted, whether ocular, skin, or otherwise.
So, if someone suggests that considering chronobiology in treatment planning is a good idea, I would agree. But if someone claims to have found definitive truth in this area, I would disagree—there is currently no consensus.
Ra Optics: In your recent paper titled “Sunlight: Between Cure and Caution,” you suggested that 20-30 minutes of midday sunlight exposure was adequate, which sparked controversy. Could you clarify this recommendation, given the complex nature of skin type and UV exposure?
Dr. Alexander Wunsch
Let’s say a Celtic individual, with Fitzpatrick Skin Type 1, has a child with someone of African descent, who has Fitzpatrick Skin Type 6. The child would not exhibit the exact characteristics of either skin type, but rather fall somewhere in between.
Suppose this child has a café-au-lait skin tone—perhaps around Fitzpatrick Type 3 or 4. What could we infer about the melanin distribution in such an individual? And more specifically, what types of melanin do you know?
Ra Optics
There is eumelanin and pheomelanin.
Dr. Alexander Wunsch
And what is peculiar about pheomelanin? It generates a significant amount of reactive oxygen species (ROS)—the exact opposite of what eumelanin does. Eumelanin acts as a buffer for ROS; it essentially stores ultraviolet radiation and gradually releases a low level of ROS over approximately 24 hours. This allows cells to accommodate the oxidative stress and respond adaptively. In contrast, pheomelanin causes a sudden burst of ROS, leading to cellular damage.
Now, if someone is exposed to sunlight and receives one Minimal Erythemal Dose (MED), how many DNA damage events would you expect to occur as a result?
Ra Optics
It would depend on how much and what type of melanin this person has, correct?
Alexander Wunsch
Not exactly. The damage occurring in the epidermis isn’t primarily determined by the amount of melanin, but rather by the number of keratinocytes in the respective skin layers. That said, the type of melanin—whether eumelanin or pheomelanin—also plays a role.
There are multiple influencing factors. First, we have the phenotype, which serves as the foundation for Fitzpatrick skin type classification. Second, the genotype determines the distribution or ratio of pheomelanin to eumelanin. Then, there’s solar callus thickness, and we must also consider the influence of chemicals and substances that may hinder the development of a solar callus.
For example, suppose someone showers frequently, like many Americans do, often twice a day, and uses shampoos and body products containing ingredients that inhibit callus formation. In that case, they scrub off the stratum corneum. This increases their susceptibility to UVB radiation. In just a few minutes, I’ve listed more than five variables that affect how an individual responds to sun exposure.
When I speak with someone in person, I can assess their skin type, skin health, and related factors to give meaningful, tailored recommendations. But when offering advice via social media, it’s impossible to accurately account for all the individual variables.
Since I’ve realized that some people have misused my messages about the benefits of sunlight, I now feel a responsibility to be much more careful in public communications. The context really matters—if someone is seriously ill, sunlight might be life-saving. For instance, a young boy treated by Auguste Rollier would likely have died within three months without heliotherapy. However, with the diligent heliotherapeutic measures, he not only survived, he was alive and kicking, and he was still healthy 10, 20, 30 years later. What I want to convey here is that everyone who issues recommendations regarding sunlight exposure has a responsibility for the people who are listening to him or her.
There are so many factors that are partially, even fully, not understood.
Take neuropsin (OPN5), for instance—a nonvisual pigment found in epidermal tissue, which derives from the ectoderm, one of the three germ layers in embryonic development. Because OPN5 is present in ectodermal tissue, it’s also found in the epidermis and dermis, as well as in other ectodermal tissues like the retinal pigment epithelium and corneal layer of the eye. This doesn’t necessarily imply it has a functional role in all these locations—you can still get a sunburn on your cornea.
If OPN5 functions locally as an accessory protection or signaling system, potentially preparing tissue for UVA exposure, it may not be limited to UVA defense. It might act as part of a broader extrapolative sensing system, common in human physiology. In such systems, blue light detection serves as a proxy for the presence of more harmful radiation, UVA and UVB. For example, melanopsin (OPN4) responds to blue light, which is less damaging, thereby minimizing the risk to the sensor itself. In this sense, OPN5 might function as an ROS alarm—a phototoxicity warning system.
However, some in the biohacking community draw incorrect conclusions. Just because there’s an alarm button doesn’t mean it should be pushed. Not every fire alarm is meant to be activated deliberately. The best course is to prevent the fire in the first place, and only press the alarm if it’s absolutely necessary.
What I want to emphasize is that this topic is complex, and giving advice carries weight and responsibility.
A good example of this complexity comes from a Danish study, which found that the longest life expectancy was associated with serum vitamin D levels between 20 and 30 ng/mL. Both lower and higher levels were linked to reduced longevity, forming a J-shaped or U-shaped curve.
It's important to note that this study predominantly involved individuals with Fitzpatrick Skin Types 1 and 2. So, when determining the optimal vitamin D level for someone, their phenotype—how their skin appears and functions—must be taken into account.
Ra Optics: You mentioned a "U-shaped curve" in relation to vitamin D levels, particularly in the Danish study involving Fitzpatrick Skin Types 1 and 2. Does this curve differ significantly across other Fitzpatrick skin types?
Alexander Wunsch
Yes, your ancestral background acts as a kind of evolutionary filter, shaped by selective pressures unique to your lineage. For instance, in regions with high UVB exposure, vitamin D deficiency is rarely a concern. However, a different challenge arises—folic acid degradation. Folic acid is essential for proper neural tube development in embryos. High levels of UVA and UVB—especially UVA, due to its deeper tissue penetration—can break down folic acid. This leads to reduced fertility in individuals who haven't developed darker skin pigmentation to protect the bloodstream from photodegradation of critical molecules.
This highlights the individuality of genotypes and phenotypes. The problem today is that there hasn’t been enough evolutionary time or selective pressure to fully optimize gene selection for modern environments. Returning to the earlier example of a Celtic individual and an African individual having a child—their offspring must now adapt to either the African or Irish environmental context. This mixed-genotype child hasn’t undergone the multi-generational stress testing required to prove resilience in a particular climate.
In Ireland, for example, someone with Fitzpatrick Skin Type 1 may tolerate sun exposure without major damage, but you can’t be certain. That’s because there was no selective pressure applied to their particular genetic makeup. Without sun exposure, there's no way to assess whether someone can physiologically "digest" that radiation. This makes predictions difficult and adds nuance. In evolutionary terms, no stress test means no guarantee, and meaningful adaptation only emerges over multiple generations.
Takeaways
Our conversation with Dr. Alexander Wunsch underscores how deeply light influences human biology and how easily its effects can be misunderstood. From melanin types and circadian rhythms to vitamin D levels and photoreceptor pathways, the variables are many and the nuances matter.
As research continues to uncover the full picture, one takeaway is clear: managing our light environment thoughtfully isn’t just helpful—it’s essential for optimizing health in today’s world.