Vitamin D by Month of the Year
Vitamin D production from sunlight follows a predictable annual cycle that most people underestimate. The cycle is largely driven by solar zenith angle — how steep the sun sits in the sky at noon. Latitudes above about 37° experience months when UVB simply doesn't reach the ground in synthesising quantity, regardless of how sunny the day feels.
January — the deepest vitamin D winter
Serum 25(OH)D reaches its annual minimum in mid-to-late January in the Northern Hemisphere. At latitudes above ~35° N, no meaningful vitamin D synthesis is occurring from sunlight — even at noon on a clear day. UVB is being absorbed by the thicker atmosphere the low-angle sun travels through. Supplementation is essential; 1,000–2,000 IU/day for most adults, higher if starting deficient.
February — nadir continues
Days are lengthening but sun angle is still low. In Boston (42° N), London (51° N), or Berlin (52° N), UVB remains below the synthesising threshold. The Mediterranean (~40° N) starts to see meaningful UVB at midday. Continue winter supplementation.
March — the equinox reboot
The spring equinox marks the return of synthesising UVB in most temperate zones. Sun angle reaches ~50° at noon. Fair-skinned adults can start making meaningful vitamin D from 15–30 minutes of face and forearm exposure at midday. Serum 25(OH)D hasn't caught up yet — there's a ~4–6 week lag between UVB return and rising blood levels.
April — 25(OH)D starts climbing
Cross-sectional data (NHANES, UK Biobank) show serum 25(OH)D beginning meaningful rise in April. Fair-skinned adults spending time outdoors at midday can start reducing supplementation dose or frequency. Darker-skinned adults at high latitudes still benefit from continuing supplementation year-round.
May — approaching peak synthesis
Sun is high, days are long. This is when 25(OH)D rises fastest. Sunburn risk also climbs sharply — a few minutes' unaccustomed sun on unprotected skin can produce a first-degree burn. Aim for pre-erythemal exposure: enough to redden slightly but not burn.
June — annual peak
Summer solstice. Sun is at maximum altitude; solar noon delivers the year's peak UVB. 5–15 minutes of arm and leg exposure between 10 AM and 2 PM produces approximately the equivalent of 1,000–2,000 IU of oral vitamin D in fair skin — more in longer sessions or larger uncovered areas. Many people meet requirements from this alone; supplementation can be reduced or paused.
July — sustained peak, deepening reserves
Serum 25(OH)D peaks in late July / early August in the Northern Hemisphere — about 6 weeks after the solstice, reflecting the pharmacokinetic delay. This is when adipose vitamin D stores are being replenished for the coming winter. See our reserves model.
August — the pivot
Serum 25(OH)D still near peak but UVB starts declining. Days shorten noticeably. This is the last month when unassisted sun exposure fully covers vitamin D needs in most people at high temperate latitudes.
September — resumption of supplementation for many
Autumn equinox. Days = nights. Sun angle drops rapidly. Above 37° N, UVB starts to lose reliability at midday. Serum 25(OH)D begins its 6-month decline. Time to restart or intensify supplementation.
October — winter is coming
For most of North America and Europe, October marks the transition into vitamin D winter. Boston (42° N) sees its last meaningful synthesising day mid- October; London (51° N) around September 30. From here, sun exposure is dietary variety and mood benefit but not a vitamin D strategy.
November — deep winter
UVB is essentially absent above 40° N. 25(OH)D falls at roughly 1–2 ng/mL per month. Supplement compliance now determines late-winter status.
December — winter solstice
Shortest days, lowest sun. 25(OH)D falling toward January nadir. Some traditional practices bear reflection: cod liver oil in Scandinavia through winter is not folklore — it's roughly 400–1,000 IU vitamin D per teaspoon plus vitamin A and omega-3, and predates commercial supplementation. Modern equivalent: 1,000–2,000 IU/day cholecalciferol for adults, more if deficient.
Southern Hemisphere
Everything above is offset by 6 months. December is the Southern Hemisphere's annual peak; June is deepest winter. Adjust accordingly for Australia, New Zealand, South Africa, and southern South America.