It's not that you need more minutes — the UVB just isn't there.
Educational only, not medical advice. Whether to supplement, and how much, is a decision for you and a
clinician — ideally guided by a blood test. OPSIN is a wellness tracker, not a medical device.
Many people assume a cold, clear winter day still tops up their vitamin D if they bundle up and get out.
For much of the world, that's physically not true — and understanding why saves you from chasing a dose
that isn't available.
The "vitamin D winter"
Vitamin D needs UVB, the shorter-wavelength part of ultraviolet. UVB is scattered and
absorbed by the atmosphere far more than UVA, and the lower the sun sits, the more atmosphere its rays
pass through. Below a certain solar elevation, essentially all the UVB is filtered out before it reaches
the ground — so there's nothing left to make vitamin D, however long you stand in it.
Above roughly 35–40° latitude (think much of the US, Europe, and higher), this creates a
stretch of winter months — commonly late autumn through early spring — where midday sun produces little
to no vitamin D. The further from the equator, the longer the window. You can still get plenty of
light for your body clock (that part still works — see our
morning light guide) — you just can't get vitamin D
from it.
Cold and clothing make it worse
Even on days when a little UVB is technically available, winter behavior removes it: you're covered head
to toe, and only your face and hands see the sky. Small exposed area + weak UVB = negligible synthesis.
Who should pay attention
Everyone at higher latitudes in winter, and especially: people with darker skin (slower synthesis),
older adults (skin makes less), anyone who covers up or stays indoors, and people with higher body weight
(vitamin D distributes into more tissue). These groups can run low even in summer, and winter compounds
it.
What to do instead
Don't chase winter sun for vitamin D. Extra time outside won't fix a UVB problem and
just adds cold-weather UV exposure to your eyes and skin for little benefit.
Consider a supplement through the vitamin D winter — the standard, honest workaround.
Typical general-wellness intakes sit well under the 4,000 IU/day adult upper limit, but the right amount
for you depends on your baseline. Get a blood test and talk to a clinician; more is not better past
repletion.
Keep getting daytime light for your circadian rhythm and mood — that benefit doesn't
take the winter off.
Bank summer sensibly. Vitamin D has a ~15-day half-life and body stores buffer you for
weeks, so a well-topped-up autumn helps — but won't carry you through a long winter alone.
OPSIN tells you the truth about your sky. On a low-UVB winter day it won't pretend a sun
session will get you there — it shows that the sun can't close the gap today and routes you to a supplement
top-up instead, then tracks your estimated level (calibrated to your blood tests) through the season.
Check today's balance → All values are modeled estimates.