In Defense of Sunlight

It’s mid-July, the weather’s too hot, and you’re probably hearing a lot of advice about sun safety. The assumption is that sunlight is a sort of adversary, responsible for everything from premature wrinkling to skin cancer.

In the midst of these frequently dire warnings, I discovered a new book proposing that the dangers of sun exposure have been exaggerated, while the benefits are greater and more diverse than most people know.

This week, instead of discussing a new study, as I ordinarily do, I’ll be reviewing the book, which appeared this June 16. It’s called In Defense of Sunlight: The Surprising Science of Sun Exposure, by journalist Rowan Jacobsen.

I first noticed this book in The Buzzed Word (“A bookshop, natural wine shop, & wine bar in Ocean City, Maryland”) and brought it to Assateague Island to read on the beach. There I applied sunscreen, sat under an umbrella, and began to wonder whether I’ve been overzealous about avoiding the sun.

The book is wide-ranging. Jacobsen digs deep into recent research, lives off the grid temporarily in the Sonoran desert, and spends time in a University of Arizona sleep lab. In the process, he develops two claims:

1. Sunlight is less harmful than most people assume.

2. Sunlight is more beneficial than most people realize.

Most of this newsletter focuses on the data behind these claims, but I want to start with some historical context that’s closely intertwined with the science.

How did we get here?

I’m old enough to remember when people regarded sunshine as a mostly unmitigated pleasure, and the only downside of getting a sunburn was the discomfort. My parents didn’t talk about skin cancer.

So, how did we reach the point that now my own kid tells me I need sunscreen every time I walk out the door?

The starting point is the heliotherapy craze of the 1920s and 1930s (”helios” is Greek for “sun”), spurred by the discovery that sunlight could cure diseases like rickets and cutaneous tuberculosis. Sanitariums and sunbathing became popular. Fortunes were made selling UV lamps and similar technologies. General Electric boasted that their Sunlamp would create a “mild sunburn” in as little as 5 minutes.

In the second half of the 20th century, a backlash emerged as skin cancer became noticeably more prevalent and studies began to implicate ultraviolet (UV) light. Both experts and the general public began to view the sun as a sort of adversary.

One could stop here, but Jacobsen suggests that data on skin cancer isn’t the only reason we’ve come to fear sunlight. He also blames technology. In a phrase reminiscent of one of Jonathan Haidt’s pet themes, he speculates that

“The more we retreat into the technological world of cities and screens, the more we need protection from the raw forces of the world.”

Also at fault, according to Jacobsen, is public health messaging (i.e., experts being overly cautious and relying on one-size-fits all advice) and, of course, capitalism. The SPF system, introduced around 1980, fueled an explosion in sunscreen sales and reinforced the notion that the sun is our enemy.

So, is sunlight as dangerous as commonly assumed? And, does sun exposure have health benefits that most of us aren’t aware of?

Part 1: How dangerous is sun exposure?

Like everything else in nature, the sun is not inherently benign. Prolonged exposure accelerates skin aging (wrinkling, loss of elasticity, etc.) and increases the risk of cataracts. On my third day at the beach, I had to stop reading because my eyes hurt, likely from a combination of reflected sunlight, salt water, and windborne particles. More intense exposure could’ve led to photokeratitis (sunburned corneas).

What we worry about most, of course, is skin cancer.

At first sight, the statistics are terrifying. The American Academy of Dermatology (AAD) website notes that

• Skin cancer is the most common cancer in the United States.

• Current estimates are that one in five Americans will develop skin cancer in their lifetime.

One in five sounds grim, but nearly 99% of these cancers are basal cell carcinomas (BCCs) or squamous cell carcinomas (SCCs), which are relatively minor. As Jacobsen puts it:

“[BCCs and SCCs] can be unpleasant and unsightly, they can leave scarring...but they rarely spread beyond the skin or result in serious health issues. They just need to be removed.”

Of course, nobody wants blemishes or scarring. Being told that you have cancer creates stress that may linger even after you’ve been pronounced “cured”. And Americans who lack health insurance may not be able to afford BCC or SCC removal unless they have access to a free clinic.

My point is not that Jacobsen downplays the seriousness of minor skin cancers. Rather, as he acknowledges, they’re an addressable problem that can be traced to excessive UV radiation, whether from the sun or from tanning beds. In the U.S., rates have been increasing in recent decades (though just how much is hard to say, because people are living longer, screening has become more prevalent, over-diagnosis occurs at early stages, and BCCs and SCCs aren’t necessarily reported to cancer registries anyway).

Far more dangerous is melanoma, which accounts for fewer than 2% of skin cancers. Among Americans, the lifetime risk of melanoma ranges from about 1 in 1,000 (Blacks) to about 3 in 100 (Whites). These figures are less scary than the AAD’s one in five statistic, but they’re still concerning.

Sun exposure and melanoma

The relationship between sun exposure and melanoma is complicated and not yet fully understood.

For instance, if too much sun causes melanoma, you’d predict that (a) sunscreen lowers risk, and that (b) people who work outdoors have higher rates than office workers do.

In fact, evidence that sunscreen reduces the risk of melanoma has been mixed – a few studies even show increased risk – and office workers have similar or higher rates of melanomas than people who work outdoors.

What’s happening here?

The current thinking is that what increases the risk of melanoma is not sunlight per se, but rather intermittent, intense exposure.

Through regular exposure to solar radiation, the skin of an outdoor worker thickens and undergoes other changes that create a buffer against melanoma, if not the less serious skin cancers. Office workers may be more prone to melanoma because their sun exposure is more likely on weekends and vacations, leaving them more susceptible to burns.

Sunburns appear to be the strongest contributor to melanoma. But the public health messaging strikes me as much more specific than what the science justifies.

In particular, we often hear that five or more blistering sunburns before the age of 20 doubles the risk of melanoma (as compared to having had no sunburns at all). This statistic comes from the Nurse’s Health Study of 2016, and leads the Skin Cancer Foundation, for example, to repeatedly air some version of the following:

Studies conducted since 2016 show that sunburns at any age increase risk, especially if experienced during the first couple decades of life. However, five is not a magical number. There’s not enough research to know how many sunburns can be experienced before risk increases.

We tend to rely too heavily the quantitative results of single studies. In fact, it’s almost assured that if one study shows that five sunburns before age 20 is the cut-off, beyond which risk increases, another study will show a cut-off of four or seven sunburns before age 18 or 25, or something like that. Each sample and methodology will yield different estimates.

One reason for this is that sunburn severity is poorly measured, owing to ambiguity in the wording of questions and reliance on participant self-report.

In short, you don’t need to start counting sunburns, or “blistering” sunburns, or whatever. Just avoid them. They increase the risk of a rare but very dangerous cancer.

So, does our society exaggerate the harmfulness of sun exposure? On the whole, I agree with Jacobsen that we do. We should still take his advice about avoiding overexposure, particularly when there’s potential for burning.

We should also worry a bit about younger folks. Surveys find that Gen Z is less concerned than older generations about sunburns and skin cancer. “Tanmaxxing” is becoming a thing, and social media sites like TikTok host a growing number of “Tanfluencers”. TikTok “lioness” really should be concerned.

Part 2: How beneficial is sun exposure?

In a book like In Defense of Sunlight, you expect the author to overstate their case, at least a little. Whether they’re an academic or a journalist, they’re trying to sell you a story.

One reason I like this book is that Jacobsen makes his case with minimal overstatement.

I realize that out of an abundance of caution, health care professionals may wince at any claim that the dangers of sunlight are exaggerated. But Jacobsen doesn’t recommend indifference. He advises (and claims to use) sunscreen to prevent burns, he suggests moderation, and, in a deliberate nod to Michael Pollan, he proposes the following:

“Get sun. Not too much. Go outside.”

Gladly. That’s what I’ve been doing all week at the beach. But what are the benefits?

We know that sunlight is essential for vitamin D synthesis. (Jacobsen notes that vitamin D supplements raise vitamin D levels but have no health benefits unless a person is severely deficient.) Sun exposure has also been linked to all sorts of desirable stuff, including better cardiovascular health, lower risk of diabetes, healthier immune system functioning, increased metabolism, higher-quality sleep, and enhanced cognition and mood.

Why?

Part of the story could simply be that sunlight reduces stress and makes us feel good. Some studies show that UVB radiation stimulates beta-endorphins, natural opioids that reduce pain while increasing feelings of relaxation and pleasure. On my second day at the beach, as the clouds began to dissipate, I felt my mood lifting, as if my brain were directly connected to the emerging sun.

I don’t want to make too much of this, because lots of things make us feel good. Plus if you fret about skin cancer, or spend a lot of time outside on one of these murderously hot July days, your mental or physical discomfort might outweigh any rush of endorphins. Let’s just say this: The fact that sunlight improves many peoples’ mood, at least under some circumstances, isn’t necessarily trivial. The stress reduction alone could make us healthier.

(Not that anything health-related happens “alone”. Strong morning sunlight, followed by sunlight throughout the day, helps keep our circadian clock working properly and improves sleep quality. Better sleep has so many downstream health benefits.)

Photorelaxation

Jacobsen has much to say about photorelaxation, which is the way that muscles – including blood vessels – relax in the presence of sunlight. As he points out, photorelaxation may stem from the effects of a molecule called nitric oxide.

In 1998, Robert Furchgott and colleagues received the Nobel Prize for discovering that our bodies produce this molecule and use it to signal the relaxation of blood vessels. The most famous application of this discovery is Viagra, which prolongs nitric oxide signaling, allowing tiny blood vessels in the penis to remain relaxed and filled with blood for a longer time.

Studies suggest that UVA radiation in sunlight stimulates the nitric oxide in our skin to act similarly on nearby blood vessels, thereby lowering blood pressure, at least in the short run.

The science on this is pretty new. I don’t think we can say anything about long-term dose-response relationships just yet. But it seems probable (or, in Jacobsen’s view, established) that time spent in the sun lowers blood pressure and thereby benefits cardiovascular health.

(Side note for male social media influencers: Exposing your penis to sunlight will not improve your erections, in spite of nitric oxide release to the blood vessels. In fact, you might experience the opposite, at least temporarily, once the sunburn kicks in.)

Anti-inflammatory effects

Jacobsen also suggests that sun exposure boosts immune system functioning. UV light is known at least to reduce the symptoms of autoimmune disorders such as multiple sclerosis and psoriasis.

How would this work? Briefly, Jacobsen proposes that UV light reduces inflammation. Specifically, it causes mild short-term inflammation that in turn triggers a prolonged and beneficial anti-inflammatory response.

This is speculative but plausible, given what we know about the skin’s natural healing process. For instance, mild exposure to UV light creates minor damage to the DNA in skin cells that the body ordinarily repairs within a few hours or more. What Jacobsen suggests is that the response to inflammation goes beyond repair to include enduring benefits.

Last but not least

Sunlight consists of many wavelengths. We mostly hear about ultraviolet (UV) light, because that’s the source of so many familiar dangers and benefits.

For instance, UVA light has longer wavelengths, penetrates the skin more deeply, and is the main culprit behind photoaging (wrinkling, sagging skin, spots, etc.). UVA is also mostly responsible for any nitric oxide effects.

UVB light, which is essential for vitamin D synthesis, has shorter wavelengths and doesn’t penetrate as deeply. It’s the primary cause of sunburns and most strongly influences the development of BCCs and SCCs.

UVA and UVB each contribute to melanoma via different mechanisms (though in an interview for this newsletter, Jacobsen pointed me to a recent National Cancer Institute study suggesting that UVA is the main contributor, while UVB might actually be slightly protective).

Jacobsen also delves into the possible impacts of infrared and near-infrared light, which are sometimes experienced as heat but have wavelengths too long to be seen.

Visible red light promotes certain kinds of healing near the surface of the skin. It’s an established treatment for oral mucositis – inflammation and sores that result from chemotherapy – and a possibly-but-not-necessarily helpful treatment for other conditions.

Near-infrared (NIR) light penetrates more deeply. (If you’re interested in exploring the benefits of some particular device for joints and muscles, I’d check first that the device generates NIR and not just red light.)

Based on a handful of recent studies, Jacobsen speculates that infrared (IR) and NIR light increase mitochondrial efficiency and energy production. The effect would be enhanced in green settings, as IR and NIR are reflected by the leaves of plants.

There’s nothing inherently wrong with this proposal, as far as I can tell, except for being somewhat speculative at the moment.

As for me laying on the beach and getting happy when the sun came out? Infrared light passes through the skull and may increase mitochondrial energy in the neurons. Perhaps our brains are, in a sense, more closely attuned to the sun than we realize.

In sum, some of the benefits of sun exposure are well-established, while others are suggested by a study or two and remain speculative until we see more data. Stay tuned.

A possible confound

Across the many studies demonstrating health benefits of sun exposure, people tend to care most about observational research showing long-term, real-world benefits. An obvious confound in these studies is physical activity.

In other words, folks who get more sun also tend to be more physically active. It’s possible that activity is responsible for some if not many of the benefits we attribute to sunlight.

I raised this concern in an email to Jacobsen earlier this week. In his reply, he acknowledged the problem, noting that “the observational evidence does not give the same confidence as an RCT. And it would be great to have stronger data all around.” He also pointed out that the opposite pattern – sunlight diminishing overall health – has not been observed:

“...the middling data in favor of more sunlight being healthy is going up against essentially a dearth of evidence on the other side. I’m not aware of any studies showing better overall health or longevity in people who get less sun exposure, and certainly none were taken into account before skin care experts started recommending sun avoidance....

Also, the better observational studies have tried to account for exercise and other confounders. Chris Dibben triangulated using both behavioral and geographic data, along with a negative control...”

Overall I’m with Jacobsen. Sunlight has many more or less clearly-established benefits. But in the observational work, statistical adjustments for activity levels are only minimally helpful.

The problem is with the way activity is measured. For instance, the Dibben study that Jacobsen mentioned, as well as several others, rely on UK Biobank data, a massive, ongoing study of more half a million older adults. In this study, people are asked about physical activities in a way that precludes a clear indoor-outdoor distinction.

Specifically, participants are asked “In a typical WEEK, how many days did you do 10 minutes or more of vigorous physical activity?” These are described as activities that make you “sweat or breathe hard”; fast cycling, aerobics, and heavy lifting are offered as examples. As you know, these and lots of other vigorous activities are sometimes done outdoors, sometimes not.

I’m not questioning Jacobsen’s broader claim here. My point is simply that we can’t rule out the possibility that some benefits of sunlight are attributable, at least in part, to the effects of physical activity.

Should we care about confounds?

In this case, maybe not. Just as Native American peoples chewed willow bark for pain relief, not knowing that the bark contains salicin (which the body converts into something close to the active ingredient in aspirin), so we can benefit from outdoor activities without fully understanding the relative contributions of sunlight versus physical activity. To extend Jacobsen’s advice a bit:

“Get sun. Not too much. Go outside (and move around).”

Final thought

If you’re honest with yourself, you live with at least some uncertainty about how your lifestyle impacts your health. You triangulate between how you feel and what experts say, and you hope for the best.

I highly recommend Jacobsen’s book. I’m certain it’s an excellent read. Less certain, at least for me, is exactly how much time we should spend outside on sunny days, and which conditions would be more or less ideal. Jacobsen argues persuasively against a one-size-fits-all approach, though he does note that it’s important to adjust gradually to sun exposure, and to preemptively avoid anything close to burning.

That strikes me as sensible. I just doubt we can know exactly how much time would be optimal, or what the benefits will be. These are uncertainties I can live with.

Thanks for reading!