The usual objection is reasonable. Tallinn spends half the year in the dark, the summer is short, and a bottle of SPF 50 in November feels like a costume. The objection still misses what sunscreen is for.
Two kinds of ultraviolet
UVB is the one you notice. It burns, it peaks in summer, it is weak here in winter, and it is what people picture when they think about sun damage.
UVA is the one you do not. It reaches deeper, it is far more constant through the year, and it passes through window glass. It is the one associated with the slow loss of firmness and the uneven tone that arrives without any single day you can blame.
That is the whole argument. You are not protecting against a Estonian August. You are protecting against the sum of ordinary days spent near a window.
What to look for
An SPF number describes UVB only. For UVA, look for the letters UVA in a circle, or “broad spectrum”, or a PA rating — without one of those, a high SPF tells you half a story.
SPF 30 applied every morning beats SPF 50 applied when you remember. The best sunscreen is the texture you do not mind wearing.
When it matters most
If you use retinol, acids or have had a peel or needling, sunscreen stops being general advice and becomes part of the treatment. Freshly renewed skin has less defence than the skin it replaced, and pigment that lands on it can stay for months.
Snow is worth a mention too: it reflects a substantial part of the light back up at you. A March ski week does more than a Tallinn July.