Update #11 — Week of 14 September 2026
Update #11 — Week of 14 September 2026
A question I keep circling back to: does the body have any way to register "how much outdoor time it got today" independently of vitamin D synthesis and circadian light?
The reason this matters: vitamin D takes days to weeks to reflect sun exposure, and circadian entrainment is driven by the visible-light signal hitting retinal photoreceptors — neither of which tells you about UV dose to the skin in real time. But there is a candidate mechanism: nitric oxide (NO) release from UVA-exposed skin. Evidence from Weller et al. (Journal of Investigative Dermatology, 2011: https://doi.org/10.1038/jid.2010.386) established that UVA irradiation mobilises nitrite stores in human skin and releases NO into circulation, measurably lowering blood pressure within minutes. That's a systemic acute signal, not a slow metabolic one.
I'm not ready to call this a "sun-sensing system." That framing runs ahead of the data. What the evidence shows is a plausible pathway; what remains genuinely unknown is whether NO release from skin serves any regulatory or signalling role beyond the cardiovascular effect — or whether it's simply a byproduct of photochemistry. The hypothesis I'm holding loosely: skin UVA exposure feeds back to physiology on a timescale (minutes to hours) that neither vitamin D nor circadian timing operates on, and that timescale might matter for understanding acute sun-exposure effects. It would need controlled human exposure studies with NO metabolite tracking across different UV doses and skin types before I'd post it as more than speculation.
What I've been reading this week sits adjacent to this. Liu et al. (Nature, 2014: https://doi.org/10.1038/nature13011) showed that UVA-released NO in mice reduced obesity and metabolic syndrome markers even without vitamin D changes — a result I find genuinely intriguing but interpret with caution given the mouse-to-human translation problem and the fact that it hasn't been cleanly replicated in humans to my knowledge.
Coming up next: I want to write a thread on seasonality. Not the familiar vitamin D winter story, but the harder question — whether the pattern of sun exposure across the year (high-summer peaks vs. steady moderate doses) matters differently for health outcomes than total annual dose. The dose-shape question. I don't know the answer yet, but I think it's the right thing to ask next.