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Vitamin D in the dark season
What latitude does to skin synthesis, and what the Nordic recommendations actually say about supplements.
This is seasonal self-care information, not a diagnosis and not treatment advice. Seasonal affective disorder is a clinical diagnosis made by a clinician; bright-light therapy is not safe to self-start for everyone (bipolar disorder and some eye conditions come first); and any measured vitamin D deficiency is treated at a clinician's dose, not a supplement-shelf one. If thoughts of self-harm are present, the step is crisis services now — the <a href="/therapists">mental health page</a> carries the national crisis lines.
Why winter removes the sun
Vitamin D is made in the skin from UVB, and UVB is the first thing the atmosphere filters out when the sun is low. In the Nordic and Baltic latitudes — roughly 58° to 65° north — the sun does not climb high enough for meaningful skin synthesis from about October to March, and at the far northern end the window is shorter still. That is not a lifestyle observation but geometry: it is the same reason the midday shadow is longer than you are all winter.
What the recommendations say
The Nordic national recommendations converge on the same figures: 10 µg (400 IU) per day for adults and children, 20 µg (800 IU) for everyone over 75, and the same 10 µg for pregnant and breastfeeding women. For people who get little sun and little fortified food, the national bodies advise a supplement through the winter months — the vitamin D supplement is the one supplement the Nordic countries recommend essentially across the board. The upper level for long-term use is 100 µg (4 000 IU) per day for adults.
Food and the fortified route
Food is a minor source at this latitude unless it is deliberate: fatty fish is the real one (see the fishing pages), plus eggs and fortified dairy, milk alternatives and margarine, which several of the countries fortify by policy. Wild mushrooms carry vitamin D₂ when they have grown in the light, but the amounts are hard to rely on. The vitamin D reference page has the full food list and the intake picture.
Testing and dosing beyond the standard
A measured deficiency is a clinician's problem, and the dose they prescribe is typically higher than the prevention figure. This page describes prevention at population level; anyone with a low measured level, osteoporosis, kidney disease or sarcoidosis is in the hands of their clinician, not a supplement shelf.
Light therapy dosing: the bright-light trials (10 000 lux, 30 minutes, morning). Vitamin D: the Nordic national nutrition recommendations. Circadian facts: the standard chronobiology literature (melanopic response, dim-light melatonin onset). The depression line: clinical diagnostic practice — seasonal affective disorder is diagnosed by a clinician. Checked 2026-09-06.
Frequently asked questions
How much vitamin D should I take in winter?
The national recommendations in this region are 10 µg (400 IU) a day for adults and children, 20 µg (800 IU) for the over-75s, and the same 10 µg in pregnancy and breastfeeding — taken as a supplement through the winter if sun and fortified food are scarce. A diagnosed deficiency is treated with a clinician's dose, which can be much higher.
Is there any point going outside in winter for vitamin D?
For vitamin D, no — the UVB is not there in the winter months at this latitude, whatever the weather. The outdoors is still worth it for everything else: daylight for the circadian clock, movement, and the cold.
The rest of the dark season
The dark season
All six winter pages, with the numbers behind them.
Light therapy: the dose that works
10 000 lux for 30 minutes in the morning — the dose the trials actually used, and how to deliver it.
Winter sleep: longer, later and how to steer it
The dark season pulls sleep later and longer; the lever that fixes it is morning light, not evening discipline.
Winter blues and when it is depression
The difference between the seasonal dip and seasonal affective disorder, and the line where help is needed.
The daylight arithmetic: how little light we actually get
Indoors is a cave even at noon in summer; outdoors on a winter day beats any lamp. The lux numbers explain why.
Track the season, not just the symptoms
TempoLife keeps sleep, steps and daylight in one picture, so the difference between your November and your June is on the chart instead of in your memory.