Here's the map, and here's what it's actually measuring.
Vitamin D synthesis in your skin needs a very specific, narrow band of UVB light. When the sun sits low in the sky, which happens automatically at higher latitudes as we move into fall and winter, that exact wavelength gets filtered out by the atmosphere almost entirely.
That's what a UV Index below 3 means, not weaker sun, a light mix that's essentially useless for making vitamin D no matter how long you stay outside in it.
Above roughly 35-37° latitude, UV Index rarely climbs above 3 from October through March. This is from Holick's 1988 research, which took actual skin samples exposed to real winter sunlight in Boston and Edmonton and found essentially zero vitamin D synthesis for months at a time.
Below about 30-35° latitude, the sun stays high enough overhead even in winter to keep the UV Index above that threshold most of the year, which is why places like Houston, Miami, and Dubai land in the year-round zone.
Find where you fall on the map. That tells you exactly how much runway you actually have left this year.
Now here's what to actually do with that information:
Invest in good blue blockers. You're about to rely on artificial light later into the evening whether you mean to or not. Protecting your evenings matters as much as chasing morning sun.
Start cold adapting now, in the morning. Don't wait until it's actually cold. Building tolerance now is what makes winter mornings easier later, not harder.
Get a full iron panel if you suspect low iron. Tired all the time, cold hands and feet, brittle hair, brain fog, these aren't just "winter is coming" symptoms to push through. Low vitamin D and low ferritin have a way of showing up together. (I use ulta labs)
You can also grab my PROTOCOLS (which has protocols to help you adapt to cold - use sauna - hydrate & more)
And speaking of windows closing: this is it for VIP pricing on the Ferritin Terrain Summit. Tonight is the last night.
Everything I just walked through, the light, the vitamin D, the iron, the whole terrain picture, that's exactly what we go deep on together at the summit.
VIP gets you lifetime access to every recording, my full Ferritin Clarity Roadmap, and close to $1,000 in gifts from our guest speakers, medical doctors, functional medicine practitioners, Chinese medicine doctors, and quantum biology experts.
Tomorrow this goes to full price permanently.
If you've been meaning to grab it, this is the deadline!
Click here or on the photo above to watch the new video now!
New guidelines just changed the definition of "low." On September 16th, the American Society of Hematology raised the diagnostic threshold for ferritin from 15 to 30. A lot of women who were told their labs were "normal" are now going to get flagged as deficient. I wrote about all of this on Substack (click here).
But the real story isn't the number, it's why so many of us got here. Chronic stress is disrupting ovulation in women in their 30s, and a lot of what gets called "perimenopause" is actually anovulatory cycles driven by stress, not the real transition.
No ovulation means unopposed estrogen, which means heavier, more unpredictable periods. More bleeding every month is more iron loss, full stop. You cannot fix ferritin with a supplement if the bleeding itself never gets addressed.
And then there's the piece almost nobody connects: sunlight, vitamin D, and Covid. Vitamin D regulates a hormone called hepcidin, the thing that locks your iron in storage.
Low sunlight, low vitamin D, and hepcidin stays elevated, so your body can't use the iron it already has.
Layer Covid on top of that: infection triggers that same hepcidin pathway, and in some people with long Covid, that disruption is still switched on years later. I go deep on the actual mechanism in the video, IL-6, hepcidin, ferroportin, the whole chain (including providing you with the actual studies to back this up).
Have a lovely rest of your day & please share these resources with your friends and family members!
Warmly,
Sarah
Not medical advice. I'm a wellness educator, not your doctor.
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