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We’ve been demonizing sunlight for years. A new book by Rowan Jacobsen digs into the science—and says we’ve got it all wrong.
“I felt as if my cells didn’t work,” writes author and journalist Rowan Jacobsen. In the spring of 2025, Jacobsen became intensely fascinated by sunlight.
As a denizen of Vermont, where winters are notoriously brutal and long, he started investigating how shifts in weather aligned with his own mood and well being. Such shifts are commonly attributed to Seasonal Affective Disorder (SAD), a condition in which people feel low during the winter months, which is believed to be caused by a dearth of light entering the eyes. But he found that he was dealing with something bigger than SAD. In fact, he believes that we might be wrongly demonizing the sun—to our own detriment.
“Like most people, I’d had it drilled into me since childhood that the sun was bad news, the primary cause of skin cancer, and that the only acceptable dosage that should be allowed to touch my unprotected skin was zero. But if we’re wired to seek sunlight, there must be a reason,” he surmised.
Looking Beyond SAD
As Jacobsen describes in his latest book, In Defense of Sunlight: The Surprising Science of Sun Exposure—released last week and recently excerpted by Slate—the seasonal changes in his body weren’t just some inchoate malaise.
He noticed how puffy he looked for one. His blood pressure would tick up a few points. And despite all the cross-country skiing he does, he’d always develop a little stomach paunch by late winter. It wasn’t just Jacobsen either. In winter, he observed that most people he knew lost their mental sharpness—and many got depressed.
In summer on the other hand, Jacobsen’s body was more lively and alert, his senses sharper, his skin healthier, and his mood brighter. It seemed like a no-brainer that it had something to do with light. “A bright spring day after a long cloudy spell was like drinking from a well after crawling through a desert,” he writes.
Though human beings don’t hibernate in the same way that bears or groundhogs do, he came to believe that our bodies do appear to enter some sort of power-saver mode when sunlight becomes less available.
The Sun’s Mood-Boosting Powers
Jacobsen’s theory is supported by science.
In 2014, a team of dermatologists at Harvard Medical School found that when our skin cells are exposed to sunlight, they produce a range of hormones that have profound effects on the body and brain. One triggers the production of melanin, the dark pigment that makes us tan and protects us from burning, and which is also a powerful antioxidant that can absorb the damage caused by the sun’s rays.
Sunlight also triggers the release of cortisol, a commonly misunderstood hormone that is often thought of as a “stress hormone” and a sign of trouble. What many don’t realize is that cortisol is only a problem when it’s chronically overproduced in response to too much stress. He writes that it’s actually the dealing-with-stress hormone—and stress can, in this case, be anything the world throws at you that requires a response, good or bad. And since sunlight initiates the production of cortisol, it makes your cells fire faster. You become more alert, you burn more energy, and you feel more focused.
A third sun-responsive compound is an endorphin that suffuses the brain with natural opioids, triggering a blissful state. The Harvard researchers focused particularly on this endorphin, and found that blocking its opioid byproducts produced withdrawal symptoms. In their paper and accompanying press release, they announced that sunlight is literally addictive.
Then in 2018, a neurology team from China published a paradigm-shifting study in Cell, showing how they developed a new technology that allowed them to analyze the makeup of individual neurons. Their paper demonstrated that sunlight hitting skin made neurons work better and improved learning and memory in mice by triggering a release of glutamate, a vital neurotransmitter. The extra glutamate reliably increased brain activity and improved mood and cognition.
Jacobsen began experimenting a bit and found that a shot of morning light led to productive, high-energy days. “It was as if some nutrient was pouring from the sky,” he writes.
Since then, other studies have confirmed the sun’s ability to boost cognition and mood in human beings through multiple pathways.
Stronger Growing Evidence
A U.K. study published in 2024 detailed how researchers attached light-sensing watches to the wrists of 89,000 volunteers to document their weekly light exposure, then followed the group for eight years to see who passed away and who didn’t. Those who didn’t get enough daylight died far more often. People exposed to the most light were 34 percent less likely to die than people who got less daylight than average. Even after adjusting for everything from exercise and diet to smoking and sleep duration, the participants with a higher dose of sun were still 17 percent less likely to pass away.
Why?
The human body evolved in an environment of abundant sunlight, and Jacobsen explains we are designed to make use of that solar energy in ingenious ways that we are finally starting to understand. It’s an elegant system that’s been fine-tuned over billions of years of existence on a sun-drenched planet. But it’s also one that’s been thrown into disarray in recent times with our retreat from the outdoor world, he notes.
Most Americans spend less than an hour per day outside; one in five virtually never sets foot beyond the built environment, Jacobsen explains. This leaves us in artificial lighting so vastly different from full-spectrum sunlight that our cells can’t make use of it. This is what chronobiologists call biological darkness.
But what about the dangers of the sun? Jacobsen posits that the dangers of sunlight loom larger in the public mind than they do in the actual data. Worldwide, sunlight is responsible for about 130,000 deaths per year, Jacobsen explains, which pales in comparison to the 20 million lives lost each year to cardiovascular disease or the 10 million lost to other forms of cancer.
Conversely, 15 scientists with expertise in these fields recently published a paper titled “Insufficient Sun Exposure Has Become a Real Public Health Problem,” in which they estimated that 820,000 deaths per year in Europe and the United States stem from sun deficiency.
Beyond the Practical: We Are Forgetting the Heavens
Our spiritual side might be the most precious thing we’ve lost as we’ve come to see the sun as an enemy, Jacobsen asserts. It casts a shadow across our entire experience of the outdoors, of nature, of the world we were born into. It can trick us into seeking the illusory safety of indoor environments with unhealthy lighting, he continues.
Underway since the Industrial Revolution, that trend is what amounts to a vast experiment to find out what happens when a species that evolved to live in the elements trades up for an indoor existence centered increasingly on the wrong kinds of rays. It will come as no surprise that the results are concerning, especially in younger generations who know nothing else.
“We are forgetting the heavens.” In a sense, we are the heavens, he elaborates.
“The difference between you and a doorstop is the energy that animates your body. Without that, no movement, no consciousness, no you. That energy comes from sunlight. Most of it fell to Earth, embedded itself in a plant, and then found its way to you. But some of it found you directly. Either way, we are lumps of mud animated by light and having a conscious experience of the universe, and we depend on constant infusions of light to keep the fun going. The good news is that—as of this writing, at least—sunlight is still free, legal, and widely available. Get it while you can.”
Jacobsen concludes that moderate sun exposure provides tremendous benefits, and one-size-fits-all recommendations seem misguided. He goes on to explain that we have compelling evidence that indoor life is deeply unhealthy. So whatever the fine points, the prescription is the same: Go outside. There’s no need to overthink it. Wear a hat, wear sunscreen, do what you gotta do, just get out there.
This post originally appeared at inc.com.
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