The sun and vitamin D. The altitude of the sun determines, not the time of day

Why the time of day matters more than how long you're outside. The advice to "take a moment in the morning sun" is nice but misleading if the purpose is vitamin D. The body's own production requires a narrow type of ultraviolet light that only reaches the ground when the sun is high enough. Low-lying sun warms, shines and can even burn - but contributes hardly anything to vitamin D.

In short

• Vitamin D is formed from UVB radiation, which is filtered out by the atmosphere when the sun is low.

• In practice, something only happens when the sun is around 30–35 degrees above the horizon, and it really only goes away above 45 degrees.

• This means midday approximately between 11 am and 3 pm Swedish summer time.

• In Sweden, the window is completely closed from mid-September to the end of March. No amount of time outdoors will help then.

• Ten to twenty minutes in the middle of the day is more than a whole morning in the slanting sun – and after that it's the shade and sun protection that counts.

It's UVB that does the work.

The skin contains a cholesterol-like substance, 7-dehydrocholesterol, which is converted into the precursor of vitamin D when exposed to short-wavelength ultraviolet light. The effect is greatest around 297 nanometers and decreases rapidly outside the band 295–315 nanometers what we call UVB.

This particular band is also what the atmosphere is best at blocking. When the sun is low, the radiation has to travel a much longer distance through the air, and the ozone layer has time to absorb almost all of the UVB on its way down. What remains is the heat (infrared), the light (visible) and a lot of UVA – radiation that ages the skin but does not produce any vitamin D.

The altitude of the sun determines – not the time of day

There is no sharp limit where production is turned off like a switch, but two rules of thumb are used in research. The generous one says that something starts to happen when the sun passes 30–35 degrees above the horizon. The stricter one and the one that is closest to how fast it actually happens says 45 degrees.

The shadow rule: if your shadow is shorter than you are, the sun is above 45 degrees. If the shadow is longer than you are tall, the yield is meager, regardless of what the clock shows.

Avoiding the midday sun is therefore excellent advice to avoid getting burned but it is exactly the wrong advice if the goal is vitamin D. The two things must be handled separately.

Your window is controlled by three things

Latitude

The further north you go, the lower the sun peaks. In Stockholm it reaches a maximum of 54 degrees at midsummer; in Kiruna it barely reaches 46. Near the equator it passes almost directly overhead all year round.

Season

The tilt of the Earth's axis causes the sun's highest point to move up and down throughout the year. This is why the season, not just the day, sets the boundary in the Nordic region.

Today's date

The window shifts a few minutes every day. A time that was correct in May is not correct in August, so it's not something you learn by heart once and for all.

This is what it looks like in Sweden

The times below are calculated using standard algorithms for the position of the sun and apply to Swedish summer time. "Core time" is the part of the day when the sun is above 45 degrees and production is at its fastest.

Midsummer (June 21)

City	Sun above 35°	Core time (above 45°)
Malmo	09:03–17:16	10:18–16:02
Gothenburg	09:10–17:18	10:31–15:57
Stockholm	08:48–16:51	10:15–15:24
Umea	08:50–16:31	10:41–14:41
Kiruna	09:07–16:15	11:54–13:28

Note that the midsummer window is about as wide in Malmö as in Stockholm. At the summer solstice, the sun's path is long and flat far north, so the number of hours above 35 degrees differs surprisingly little. It is the core time the hours above 45 degrees – that shrinks northward, from almost six hours in Malmö to just over one and a half in Kiruna.

Mid-August

City	Sun above 35°	Core time (above 45°)
Malmo	10:07–16:17	11:48–14:36
Gothenburg	10:21–16:11	12:28–14:04
Stockholm	10:06–15:38	—
Umea	10:33–14:52	—
Kiruna	11:46–13:40	—

Two months after Midsummer, core time has already disappeared north of Gothenburg. The difference between June and August is therefore much greater than it seems.

The season in its entirety

City	Sun above 35°	Sun above 45°
Malmo	March 22 – September 21	April 18 – August 25
Gothenburg	March 27 – September 15	April 24 – August 18
Stockholm	April 1 – September 11	April 29 – August 13
Kiruna	April 24 – August 18	June 8 – July 4

Outside of these dates, it doesn't matter how many hours you spend outdoors – the sun is simply never high enough. It's usually called vitamin D winter, and in Sweden it lasts about six months.

Five things that change the bill

• Clouds. Thin or scattered clouds make unexpectedly little difference and in some situations can even increase radiation through scattering. Only with dense, complete cloud cover does UVB seriously decrease. There is no simple formula expect a lot of variation.

• Skin tone. Melanin acts as a built-in filter. Model calculations indicate on the order of ten minutes for very light skin and three to four times as long for dark skin under the same conditions.

• Altitude. UV radiation increases by approximately ten percent per thousand meters, and for UVB in particular the increase is even steeper.

• The surface. Fresh snow reflects up to 80 percent of UV radiation and can practically double exposure. Sand is around 15 percent, grass and soil less than ten.

• Sunscreen. In the laboratory, factor 30 blocks about 97 percent of UVB. In reality, most people apply too thinly and too infrequently, and reviews of studies show that regular use has little or no effect on vitamin D levels. Forgoing sunscreen for the sake of vitamin D is therefore a bad idea.

How to do it practically

Aim for midday in the summer months, keep your session short, and finish well before you get red. Bare skin on your arms and legs provides much more surface area than your face and hands, and surface area means more than minutes. If you burn, you haven't gotten any more vitamin D production levels off long before your skin takes damage, while the damage continues to increase.

During the Swedish vitamin D winter, the body has to rely on the stores from the summer, on the diet – fatty fish, eggs, fortified dairy and fat products and for many people on supplements. The Swedish National Food Agency has current recommendations for different age groups; it is more reasonable to follow them than to chase the sun in November.

And a reminder that can't be ignored: UV radiation is the most important cause of skin cancer. For this reason, dermatological societies do not recommend sunbathing as a source of vitamin D. If you want to optimize your levels, supplementation is both safer and more predictable - the sun is a bonus, not a strategy.

Contribution

For six months, Swedes are dependent on summer supplies, their diet – fatty fish, eggs, fortified dairy and fat products – and supplements. The Swedish National Food Agency sets the recommended intake at 10 micrograms per day for most people, and 20 micrograms for everyone over 75. Supplements are especially recommended for children under two years of age, for everyone over 75, and for those who rarely eat fish or have very limited sun exposure.

All the best, Kennet Båth

About the calculations

The solar altitudes are calculated for the respective location coordinates using established solar position algorithms and are reported in local time (summer time). The limits of 35 and 45 degrees are rules of thumb taken from the literature, not sharp physical limits - production increases gradually with solar altitude rather than being switched on and off.