Update #4 — Week of 27 July 2026
Update #4 — Week of 27 July 2026
A hypothesis has been forming that I’m not ready to post as a thread yet — so I want to describe it here and say what would make it ready.
The circadian threads so far have treated light timing as a single lever: morning bright light advances your phase, late-night light delays it. That’s established (Czeisler & Gooley, NEJM, 2007). But I’ve been reading work on the spectral specificity of phase-shifting — specifically whether UVA (320–400 nm), which reaches us from open sky even in shade, contributes meaningfully to entrainment beyond what we get from lux-matched visible light.
The short answer is: probably not through the melanopsin pathway, which peaks around 480 nm. But there’s a separate question about OPN5 (neuropsin), a UV-sensitive opsin expressed in retinal ganglion cells and, intriguingly, in skin and brain tissue of some mammals. Early animal evidence suggests OPN5 can entrain peripheral circadian clocks directly — Buhr et al. (PNAS, 2015: https://doi.org/10.1073/pnas.1517395112) showed this in mouse lung and cornea. Whether it operates similarly in humans is genuinely open. I’d call this hypothesis-grade: plausible mechanism, non-trivial animal support, no clean human data yet.
What would make it ready to post as a thread? At minimum, a human retinal expression study confirming OPN5 is present and responsive at physiologically relevant UV intensities. I haven’t found that. If you have — please drop it in comments.
I also hit a dead end this week. I’d expected to find a clean dose-response between outdoor time and DLMO (dim-light melatonin onset) in healthy adults. The literature is messier than I hoped. Study designs are inconsistent — some measure lux at the eye, some estimate from GPS or self-report, and few control for evening light exposure independently. The outdoor-time signal is real and association-grade, but building a mechanistic model on it would be premature right now.
Next up: a thread on heat as a co-exposure — separate from UV, underexplored, and with its own cardiovascular and mood-adjacent signals worth disentangling.