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Vitamin D and tiredness
Low levels are common at these latitudes, but D is rarely the whole story on its own.
Vitamin D deficiency is real and common in Northern Europe — the sun angle cannot make D in the skin from roughly October to March — and it causes tiredness, muscle aches and low mood. But it is also over-blamed: D is often low alongside whatever is actually wrong, and fixing it while ignoring sleep, iron and mood changes nothing.
The check is a 25-hydroxyvitamin D blood test, best taken at the end of winter when stores are lowest. If it is low, the national guidance dose is boringly standard — typically 20-50 micrograms a day for a few months, then a maintenance dose. There is no evidence that huge doses beyond correction do anything extra.
Check first
A 25-hydroxyvitamin D test at the end of winter.
Reference, not medical advice. If tiredness is progressive, unexplained, or comes with weight loss, fevers, night sweats or breathlessness, see a clinician. Nothing here diagnoses.