Yes, for most people. Summer sun raises vitamin D levels but rarely enough to cover the rest of the year, and levels peak in August and September before falling until February. Stopping in summer means giving up the one stretch where daily intake adds to a high number instead of digging out of a low one.
Key takeaways
- US vitamin D levels peak in August and September, then decline for roughly six months.
- Circulating vitamin D has a half-life of about 15 days, so summer sun does not carry forward.
- Roughly 70% of Americans are low even at the best time of year for it.
- Sunscreen at SPF 8 or higher blocks most of the UV that makes vitamin D.
- Above about 40°N, skin stops producing any vitamin D for part of the year.
How much vitamin D does summer sun actually give you?
Less than most people assume, and it varies enormously between two people standing in the same yard.
The NIH Office of Dietary Supplements notes that sunscreens at SPF 8 or higher block the UV wavelengths that produce vitamin D, though in practice people rarely apply enough or reapply often enough to shut it down completely. Skin tone matters too: melanin competes for the same UV, so darker skin can need up to ten times longer to make the same amount.
Add an indoor job, a commute, and the reasonable habit of not baking in midday sun, and a lot of people get through a whole summer without much to show for it. That's the backdrop to roughly 70% of Americans being low. Not a winter number. A year-round one.
When do vitamin D levels peak?
August and September. A 2013 analysis in PLOS ONE pooled 3,440,710 blood samples sent to the Mayo Clinic from across the United States and tracked them week by week across 287 consecutive weeks. Levels peaked in August and bottomed out in February, every year in the dataset.
3.44 million serum samples. Peak in August, trough in February.
The peak lands about eight weeks after the longest day of the year, for the same reason the hottest weeks of summer arrive well after the solstice. Your body accumulates vitamin D gradually across every sunny month, so the total keeps climbing after the sun has already started dropping. Then it runs the same process in reverse.
Vitamin D levels peak in late summer and bottom out in February, about eight weeks behind the sun.
In the Cardiovascular Health Study, which tracked 2,298 adults, the average gap between an individual's yearly high and yearly low was 9.6 ng/mL. Men, people at higher latitudes, and more physically active people all swung further. Which means the people who spend the most time outside all summer have the furthest to fall.
How long does vitamin D stay in your body?
About two weeks, in the form that gets measured.
The NIH puts the circulating half-life of 25(OH)D, the marker used to assess vitamin D status, at roughly 15 days. That's long for a nutrient. It's nowhere close to long enough to bank a summer.
A 15-day half-life means that without new sun or intake, about half of what you have circulating is gone in two weeks. Then half of what's left. A great July does not fund a February.
Can you stop in the summer and start again in the fall?
You can, and it's the most common pattern there is. It's also backwards.
Because levels move gradually rather than in steps, what you do now sets the height you start falling from. Taking it through August and September means adding to your annual high. Restarting in November means climbing back from a number that's already dropped for two months, using the same 15-day half-life that made the summer break feel harmless.
Same total effort. Different curve.
Does where you live change the answer?
It changes how brutal the second half of the year gets. A 1988 study by Webb, Kline and Holick exposed human skin to real sunlight on cloudless winter days at four latitudes and measured what it produced. It's where the term "vitamin D winter" comes from.
| Latitude | Example | Months with no measurable vitamin D production in skin |
|---|---|---|
| 18°N | San Juan, Puerto Rico | None. Midwinter sunlight still works. |
| 34°N | Los Angeles, Atlanta | None. Midwinter sunlight still works. |
| 42.2°N | Boston, Chicago, Detroit | November through February |
| 52°N | Edmonton, London | October through March |
The Linus Pauling Institute at Oregon State puts the practical cutoff around 40 degrees, north or south, where there isn't enough UVB for skin synthesis from November into early March. Draw that line and it runs near Philadelphia, Columbus, Indianapolis, Denver, and just north of Sacramento.
Above it, you get a hard stop for part of the year. Below it, still a steep drop, just without the full shutdown. And all of this assumes clear skies and bare skin, which is not how anyone spends a January. More here: can you get enough vitamin D from the sun?
What to do while your levels are still high
Test now if you're going to test. A reading taken this month is close to your annual maximum. A reading taken in February is close to your annual minimum. Same person, very different number. If a doctor once told you your level looked fine, check what month the blood was drawn.
Know what running low looks like. Most people never test and never connect the two. Here are the signs worth watching for.
Don't take the summer off. The NIH recommended dietary allowance is 600 IU per day for most adults up to age 70, with common supplemental amounts sitting higher. Full range: how much vitamin D should you take.
Then take it daily. A 15-day half-life means consistency matters more than dose size. Sporadic use produces a sawtooth, not a level. That's the reason we built D3X as a liquid with a marked dropper and a subscription timed to a 30-day bottle: the hard part of vitamin D was never the science, it was remembering to take it.*
Your level is about as high as it gets right now. Everything from here is subtraction until the sun comes back.
Sources
Kasahara AK, Singh RJ, Noymer A. Vitamin D (25OHD) Serum Seasonality in the United States. PLOS ONE, 2013;8(6):e65785.
Webb AR, Kline L, Holick MF. Influence of season and latitude on the cutaneous synthesis of vitamin D3. Journal of Clinical Endocrinology & Metabolism, 1988;67(2):373-378.
Kroll MH et al. Seasonal Variation in 25-Hydroxyvitamin D Concentrations in the Cardiovascular Health Study. American Journal of Epidemiology, 2012.
National Institutes of Health, Office of Dietary Supplements. Vitamin D Fact Sheet for Health Professionals.
Linus Pauling Institute, Oregon State University. Vitamin D and Skin Health.
Last updated: August 2026
About D3X. D3X makes one thing: a clean liquid vitamin D3 + K2 (2,000 IU D3 / 100 mcg K2) in an olive-oil base, third-party tested and made in the USA. No seed oils, fillers, or sweeteners. Learn more →
Frequently Asked
Do you need to take vitamin D in the summer?
For most people, yes. Summer sun raises levels but rarely enough to cover the rest of the year, and roughly 70% of Americans are low even at the seasonal peak. Sunscreen, indoor work, and skin tone all reduce how much you actually produce.
When do vitamin D levels peak?
In the United States, serum vitamin D peaks in August and September and reaches its lowest point in February. The peak lands about eight weeks after the summer solstice.
How long does vitamin D stay in your body?
Circulating 25(OH)D has a half-life of about 15 days according to the NIH Office of Dietary Supplements. Without new sun or intake, roughly half of what is circulating is gone in two weeks.
Can you stop taking vitamin D in the summer and start again in the fall?
You can, but it works against you. Taking it through late summer adds to your annual high, while restarting in the fall means climbing back from a level that has already been falling for weeks.
At what latitude do you stop making vitamin D from sunlight?
Around 40 degrees north or south. Webb, Kline and Holick found skin in Boston at 42.2°N produced no measurable previtamin D3 from November through February, and in Edmonton at 52°N that window ran October through March.
