I first became interested in this topic due to a podcast by two evolutionary biologists, where they proposed their hypothesis, based on experiences studying in South America, that relatively short period in the shade could return the ability to withstand relatively long periods in the sun, without burning, such as a ten minute break in the shade providing another hour of sun exposure without threat of burn.

They connected this hypothesis to some degree with evolutionary theory, along with many other practical propositions found in their book, The Hunter Gatherer's Guide to the 21^st Century. The two have a history of pointing out practical implications of evolutionary theory, in the form of predictive hypotheses that withstand tests, derived from the gold-standard, hypothetico-deductive scientific method. Their proposition about sunburn was the first that I had ever heard, regarding an actual insight into the body's natural ability to deal with the sun, as opposed to exogenous solutions.

As I tried out their hypothesis myself, I began to have suspicions of my own, as I collected preliminary data. First, I tried to test the theory by time alone, and found that it was implausible to know what length of time could be chosen for exposure, or for shade, to produce the effect. While the effect seemed real, the practical implementation was lacking, in that knowing it was the case was insufficient for successfully applying the idea to prevent sunburn.

I therefore began looking for alternative means of guaging the sunburn proximity of skin, as a means of determining from the ground up what lengths of time should be used to test their proposition. This is how I noticed the patterns in skin temperature, which I had never heard any mention of before then. Most people know that a fever of 103°F is serious business, but what about a forearm of 105°F? What about 107°F? It seemed to me that I could detect even by hand a reliably distinguishable difference in temperature during sun exposure of tan areas of skin compared to pale, with pale skin reaching the nociceptor (pain) threshold in the feeling hand.

The far end of these suspicions led me to wonder if sunburn really is even caused by the sun at all, but rather caused by the temperature that our skin reaches when unprepared for the sun. We do know that skin damage, of any sort, is related to many of the negative outcomes that have been associated with sun exposure. Is it not possible that the sun is actually not the cause of these effects, but rather that sunburn represents a basic form of heat damage as paler skin reaches temperatures it should not, and indeed would not, if it were tanned to the appropriate level for the average sun strength at that time of year.

It's possible, if this is true, that the dangers we associated with the sun, which lead us to cover up and hide all year until that one summer weekend when we blast our skin with a sudden and impossible increase in UV, are actually dangers created by this very misunderstanding of the sun as dangerous. It's possible that hypernovelty is to blame, wherein our behavior has diverged from the ancestral pattern, for which we are adapted.

It has seemed to me that the modern perspective on the sun does not make sense with evolutionary theory. Belief that the sun is inherently dangerous is ubiquitous. Johns Hopkins states,

"A suntan represents the skin's response to injury from the sun. A small amount of sun exposure is healthy and pleasurable. But too much can be dangerous. Measures should be taken to prevent overexposure to sunlight... The best way to protect yourself against the damaging effects of the sun is to limit exposure and protect your skin. The best way to prevent sunburn in children over 6 months old is to follow these tips from the American Academy of Dermatology: Generously apply a broad-spectrum water-resistant sunscreen with an SPF (Sun Protection Factor) of at least 30 to all exposed skin. Broad spectrum means the sunscreen protects you from both UVA and UVB rays. Re-apply about every 2 hours and after swimming or sweating..."

Here, the CDC starts its explanation of sunburn by directly tethering it to the lack of sunscreen:

"Sunburn is an often painful sign of skin damage from spending too much time outdoors without wearing a protective sunscreen."

Sunscreen appears to be considered the de facto essential means by which sun exposure can be made healthy. This is odd, considering our evolutionary lineage probably lost its fur around 1.2 million years ago (New York Times), leaving our skin generally exposed to the sun a large part of the day for the duration of our hunting gathering ancestry, before agriculture led to larger amounts of time spent indoors.

According to the EPA, "one American dies from skin cancer every hour." How can this be happening in generations that have some of the least total sun exposure ever? "Melanoma, the most serious form of skin cancer, is now one of the most common cancers among adolescents and young adults ages 15-29." BusinessWire reports data that, "While 75% of Millennials and Gen Xers are mindful of their sun exposure, only 19% use sunscreen year round," a strange statistic to provide given zero mention of latitude. The question then becomes, were ancestors who were not mindful of their sun exposure nor using sunscreen also exhibiting these rates of melanoma?

In many places, the term "overexposure to the sun" is never defined specifically, except occasionally by relativity to sunburn, and the implications of the fact that skin tanning increases the threshold to burn are never properly integrated with the term. Instead, it is claimed that the negative qualities of sunburn apply just as well to skin being exposed to the sun without burn. While burn-causing overexposure can be linked to many acute negative effects, there is little reason to directly connect the sun exposure all the way to long-term cancer outcomes, without first having investigated the manner in which those short-term effects are modulated.