UV400 sounds like a spec sheet number invented to sell sunglasses, but it refers to something specific: the lens blocks light wavelengths up to 400 nanometres, which covers the entire UVA and UVB range that reaches the earth's surface. Anything labelled UV400 is, by definition, blocking essentially all the UV radiation your eyes would otherwise be exposed to.
Darker doesn't mean safer
This is the part most people get backwards. A dark lens without UV protection is arguably worse than no sunglasses at all — the tint makes your pupils dilate to let in more light, but if the lens isn't filtering UV, more of it reaches your retina than if you'd worn nothing. Tint is about comfort and glare; UV400 is about protection, and the two aren't related.
Why it matters beyond sunburn
Skin gets talked about a lot in sun-protection conversations; eyes rarely do. Cumulative UV exposure over years is a known contributor to cataracts and a condition called pterygium — a growth on the surface of the eye common in people who spend a lot of time outdoors without protection. It's slow and invisible until it isn't, which is exactly why it's easy to ignore.
How to actually check
Every pair of sunglasses we sell states UV400 protection on the frame — it's standard across our whole sunglass range, not an upgrade. If you're buying elsewhere, look for "UV400" or "100% UV protection" printed on the lens or a sticker; if a pair only says "UV protective" with no number, it's worth asking directly, since that phrasing alone isn't a guarantee.