So do we really need to be defending the sun in 2026? Jacobsen is quick to clarify that he’s not in the anti-sunscreen, get-ready-to-lay-out-with-him camp, but also argues there’s no need to panic about our culture’s current wave of UV enthusiasm. His book is a tight 199 pages (268 if you count the glossary, resources, notes, and index at the end) and cites nearly as many studies—many published in peer-reviewed journals like the Journal of the American Medical Association, The Lancet, and even the Journal of the American Academy of Dermatology—that point toward prolonged sun avoidance being quite dangerous.

“Sun deprivation was linked to heart attack, stroke, diabetes, dementia, depression, Parkinson’s, myopia, respiratory infections, certain cancers, and autoimmune conditions such as multiple sclerosis and inflammatory bowel disease,” he writes in the introduction. “That evidence had been showing up in various observational studies for decades, but it had been growing stronger as the tools got better.” At one point, he cites a study published in the Journal of Internal Medicine that found nonsmokers who avoided the sun had about the same life expectancy as smokers who embraced the sun. “Avoidance of sun exposure seems to be a risk factor of magnitude similar to smoking in terms of life expectancy,” the authors concluded.

“Sun deprivation” isn't an officially defined term; most of the studies Jacobsen cites look at self-reported time spent in (or avoiding) the sun alongside vitamin D levels and health outcomes. Jacobsen tells me his "not official nor a prescription, just one informed guy’s back-of-the-envelope rubric" is that you're probably “deprived” if you're not getting enough sun to maintain a vitamin D level of 20 ng/ml or above (without supplementation). The amount of time you'd need to spend outside to reach those levels will depend on the weather, season, your skin tone, and location. “But as a general rule, it’s never about getting lots of sunlight, it’s about making sure you get more than none,” he says.

On the flip side, spending some time in the sun is linked with, well, less of all the risks mentioned above: less depression, lower blood pressure, a lower risk of diabetes, enhanced wound healing, less heart disease, better mental health, less fatigue, fewer autoimmune disorders…essentially, being outside seems to be quite good for you. It used to be widely accepted that this was because UV rays help your body produce vitamin D. But, Jacobsen and many scientists now argue, if it was just a vitamin D thing, then you should be able to pop a pill and stay inside forever. Instead, more recent studies have found that, while vitamin D supplements do, indeed, raise vitamin D levels, those artificially-achieved higher levels don’t necessarily correlate with better health outcomes.