Life
Sun’s Out, Buns Out
For decades, we’ve demonized the sun. Turns out we were mostly wrong.
June 16, 2026 10:33 AM
Joe Sohm/Visions of America/Universal Images Group via Getty Images
The following is an excerpt from Rowan Jacobsen’s forthcoming book, In Defense of Sunlight: The Surprising Science of Sun Exposure , out from Simon & Schuster June 16. It has been edited and condensed for Slate.
In the spring of 2025, I became intensely fascinated by a topic that, for the last seven years, had become increasingly present in my mind. That topic, which would soon consume my every waking thought and which I would eventually turn into a book , was sunlight.
The curiosity stemmed from an ongoing observational study of a single subject: me. I live in Vermont. Gorgeous summers, brutal winters. Over the years, I started to notice what a profound effect the different seasons had on me. Basically, winter sucked. This was not simple Seasonal Affective Disorder, a common condition in which people feel low during the winter months, and which is believed to be caused by a dearth of light entering the eyes. I didn’t feel depressed in winter. I felt as if my cells didn’t work.
In summer, my senses were sharper. My thinking was sharper. I ate less but did more, as if nutrients and oxygen were flowing more freely. My skin felt snappier, especially once it got a little color. And I was way happier. Small things seemed deeply pleasing.
And then, when the days grew shorter and the temperature dropped, it all went away. This was not due to lack of exercise. I go for a cross-country ski every decent day of winter. I probably get more focused cardio workouts in the snowy months. But it didn’t matter. In winter, my brain and body never felt like they were firing on all cylinders.
It also wasn’t just some inchoate malaise. I could see the difference. In the mirror, I looked a little bit puffy. Despite all the skiing, I’d have a hint of a pouch by late winter. My blood pressure would creep up a few points.
And it wasn’t just me. Most people I knew lost their sharpness in winter. Many got depressed. We all commiserated about the short days. Human beings don’t hibernate like bears or groundhogs, but our bodies were clearly going into some sort of power-saver mode when the sun withdrew.
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. But I never thought much about the mechanism until 2014, when a team of dermatologists at Harvard Medical School confirmed the sun’s mood-boosting powers . It turns out 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 mop up the damage caused by the sun’s rays.
Sunlight also initiates the production of cortisol, the hormone of activity. It makes your cells fire faster. You get more alert, you burn more energy, and you feel more focused. People think of cortisol as the “stress hormone” and think it’s a sign of trouble, but cortisol is 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. Cortisol is only a problem when it’s chronically overproduced in response to too much stress. I actually think of cortisol as the “in the zone” hormone. When you are in a tennis match, ready to pounce on a weak serve and fire a cross-court winner, that’s cortisol talking. When you are crushing charades at game night, that’s also cortisol. It’s the hormone of high performance, of rising to the occasion, and it naturally rises in the morning, but happens faster when sun kisses skin.
A third sun-responsive compound is an endorphin that suffuses the brain with natural opioids, triggering a blissful state. That one was the focus of the Harvard researchers, who found that blocking the opioids produced withdrawal symptoms. In their paper and accompanying press release, they announced that sunlight is literally addictive.
To me, the discovery had profound evolutionary implications. Biological reward systems evolve to encourage beneficial behavior. So the discovery was somewhat mystifying. 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.
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But that wasn’t how the Harvard doctors saw it. They acknowledged that the discovery implied some “potential evolutionary benefit … that reinforces UV-seeking behavior,” but they didn’t want to touc…
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