The sunlight conversation
nobody's having.

Inspired by Dr. Roger Seheult's interview on The Diary of a CEO with Steven Bartlett.

I've always believed in getting as much sun as I reasonably can. It's instinctive — the way good light feels on skin, the way a bright morning changes your mood before you've even had coffee. So when I came across Steven Bartlett's interview with Dr. Roger Seheult on The Diary of a CEO, I expected to nod along to things I already believed. What I got instead was something more useful: an actual physician — board-certified in internal medicine, pulmonology, critical care, and sleep medicine, and co-founder of MedCram — laying out why sunlight matters, with more nuance than the wellness-internet version of this conversation usually allows.

I want to walk through what's actually well-supported here, because the truth is more interesting than the slogan version, and because getting it right matters more than getting it punchy.

The story that opens the conversation

Seheult opens with a real case: a 15-year-old boy, referred to in his telling as Henry, undergoing chemotherapy for lymphoblastic leukemia — a blood cancer. Chemotherapy suppresses the immune system, which is precisely the danger point in his treatment. The story, as Seheult recounts it, is about how factors around sunlight and immune function became part of the boy's care conversation during that vulnerable period.

I'm not a doctor, and I want to be careful here: this is one physician's anecdote, not a clinical trial, and it's not a substitute for an oncologist's treatment plan. But it's the entry point Seheult uses to make a broader argument — that sunlight does far more in the body than the one thing most of us learned in school, which is that it makes vitamin D.

Sunlight is not one thing — it's a spectrum

This is the part of the interview I found most clarifying. Seheult breaks down sunlight by wavelength, the way you'd break down sound by frequency. Ultraviolet light — the shortwave end — is what triggers vitamin D synthesis in skin, but it doesn't penetrate deeply. Infrared light, the longer wavelength, penetrates much further into the body and, in Seheult's account, interacts with mitochondria — the energy-producing structures inside our cells — in ways that go well beyond vitamin D production. He connects this to mitochondrial energy output, oxidative stress regulation, and even melatonin produced inside cells as a localized antioxidant, distinct from the melatonin your brain releases at night for sleep.

Whether you supplement with vitamin D or not — and Seheult himself says he does, while still maintaining sunlight gives you considerably more than the vitamin alone — the bigger claim is that we've spent decades reducing "the sun" to "the vitamin," and missed most of what's actually going on.

The real nuance on skin cancer

Here's where I want to slow down, because this is the part most retellings of this interview flatten into something it isn't.

Seheult does not argue that sunlight has no relationship to skin cancer, and he does not argue that sunscreen is some kind of fraud. What he does raise — and what's genuinely worth sitting with — is a more specific and uncomfortable question: are we certain that total sun avoidance, indoor living, and reflexive total-coverage sunscreen use are net-positive, when population data shows indoor workers and people with minimal sun exposure tend to have worse outcomes on a range of measures, not better?

That's a real, open conversation inside actual medical research — it's not the same as "sunscreen is a lie." The honest version of this is: moderate, sensible sun exposure appears to support real physiological functions that total avoidance does not, while still leaving sunburn and cumulative overexposure as genuine risk factors for skin cancer.

What I'm not going to tell you

I want to be direct about something, because I almost wrote a different, sloppier version of this article. I'd seen the claim that coconut oil functions as a natural SPF 4 sunscreen, and I wanted to repeat it here as a kind of practical takeaway. I'm glad I checked first, because it isn't true in any way that matters. The claim traces back to a single small lab study from 2007 measuring UV absorbance in a test tube — it was never validated as real-world protection, and dermatology research since has found coconut oil offers negligible measurable protection against UVB rays and essentially none against UVA, the wavelength most associated with skin aging and cancer risk. I love coconut oil for plenty of reasons. Sun protection isn't one of them, and I'm not going to pass that claim along just because it's appealing.

That's the difference I'm trying to hold onto here: there's a real, fascinating, under-discussed science of sunlight and human health, and there's a separate pile of wellness-internet shortcuts that borrow that credibility without earning it. The first is worth your attention. The second just makes the first harder to take seriously.

What actually seems worth doing

A few things from the interview that are genuinely well-supported and don't require throwing out sunscreen or common sense:

Morning light, early and consistently — Seheult discusses the circadian and dopamine-related benefits of getting outside shortly after waking, before reaching for caffeine. The mechanism is about anchoring your body's clock, not about UV dose.

Time outdoors generally, including on cloudy days — infrared and visible light still reach you through cloud cover; the data Seheult cites shows mortality differences tied to sunlight exposure even in lower-UV conditions.

Time among trees — he cites the Green Heart Study out of Louisville, which found that urban tree planting measurably reduced inflammation markers in residents. That's a genuinely interesting, separate thread — phytoncides and immune function — worth its own read.

Treat sunscreen and sun exposure as not mutually exclusive — protect against burning and cumulative overexposure, without defaulting to zero sun as the only safe amount.

Full interview: "Vitamin D Expert: The Fastest Way To Dementia & The Dangerous Lie You've Been Told About Sunlight!" — The Diary of a CEO with Steven Bartlett, July 17, 2025. Watch on YouTube →