Rowan Jacobsen is making a direct case that sunlight does far more than brighten a day. In an excerpt from his forthcoming book, In Defense of Sunlight: The Surprising Science of Sun Exposure, he argues that sun exposure changes mood, energy and the way the body works.

The timing matters because the excerpt lands ahead of the book’s June 16 release from Simon & Schuster. Jacobsen says he turned a growing fascination with sunlight into the book after becoming intensely interested in it in the spring of 2025, even though he says the idea had been building in his mind for the last seven years.

His case begins with his own body. Jacobsen, who lives in Vermont, says winter there is brutal. He says he did not feel depressed in winter, but felt as if his cells did not work. In summer, by contrast, his senses felt sharper, his thinking was clearer, he ate less but did more, and he felt “way happier.” Small things, he says, seemed deeply pleasing once the days were long and the light was strong.

He frames that difference as more than a mood swing. Jacobsen says sunlight exposure in skin cells produces a range of hormones with profound effects on the body and brain. One of them drives melanin production, the pigment that makes people tan, helps protect them from burning and also acts as a powerful antioxidant. Sunlight also initiates cortisol, which he says makes cells fire faster, leaves people more alert, helps them burn more energy and makes them feel more focused.

The argument is not built only on his own winter and summer observations. Jacobsen points to a 2014 team of dermatologists at Harvard Medical School that, he says, confirmed the sun’s mood-boosting powers. He uses that finding to push against the idea that sunlight is mainly something to avoid, not something with measurable benefits.

Still, he does not present a clean seasonal morality tale. Jacobsen says he goes for a cross-country ski every decent day of winter and gets more focused cardio workouts in the snowy months, yet he also says his brain and body never felt like they were firing on all cylinders. By late winter, he says, he looked puffy in the mirror, had a hint of a pouch and saw his blood pressure creep up a few points.

That is where the excerpt lands with force: not on a claim that winter is bad in every way, but on the idea that sunlight may be doing quiet biological work people underestimate. The book’s release on June 16 will show whether Jacobsen’s mix of self-observation and research can persuade readers that what feels like a simple preference for summer is also a case for sun as a physical and mental force.