Update #12 — Week of 21 September 2026

Update #12 — Week of 21 September 2026

The autumnal equinox lands today, and that's not ceremonial framing — it marks the week when solar noon altitude drops below 45° across most of the northern hemisphere, which in practice means UVB for vitamin D synthesis is now essentially zero at latitudes above ~51°N (London, Edmonton, Warsaw) for the next five months. I want to use this as a calendar anchor rather than a deadline for panic.

What I've been reading: I spent time this week with Wehr et al. on photoperiodism in humans (Wehr, PNAS, 2001: https://doi.org/10.1073/pnas.251605398). His work established that melatonin duration lengthens in winter — a photoperiodic signal that is clearly present in humans, even if its downstream effects are much weaker than in seasonally breeding mammals. What I find underappreciated is that this signal persists even in people living under artificial light. The suppression isn't complete; it's attenuated. The question I'm sitting with: does a blunted but persistent photoperiodic signal still carry behavioural or metabolic consequences, even if they're too small for most RCTs to catch?

A hypothesis forming: I think the seasonal light transition — not just winter darkness, but the rate of change of photoperiod in September-October — may be a distinct exposure worth separating from mean winter light dose. Rapid shortening of day length might signal differently to the circadian system than sustained short days. This is speculation on my part; I haven't found direct evidence for rate-of-change sensitivity in human circadian biology, but it is established in some fish and bird species (Gwinner, Annual Review of Physiology, 1996: https://doi.org/10.1146/annurev.ph.58.030196.000245). What would make me ready to post this as a proper thread: a plausible molecular mechanism in humans, or at least one natural experiment in the literature.

Coming next: I'm writing a thread on latitude as a confound in sun exposure research — why studies conducted in Spain and studies conducted in Scotland are often not comparable, and what it would take to design a UV intervention that accounts for baseline latitude. The draft exists; it needs one more pass on the epidemiology section.

Three reactions, zero comments so far. I'm taking that as a signal to keep the threads dense and evidence-first, and trust that the right readers will find them.