[yixu34]

I'd suggest checking out Lab Muffin Beauty Science's videos and blogs. The tl;dr is that ideally yes, not because modern sunscreens break down (they're generally pretty photostable), but because your skin cells move over time and create gaps in coverage.

Side note: you can also buy a UV camera on Amazon that shows your suncreen application coverage!

[layman51]

There’s also UV exposure stickers. I haven’t used them, but the idea is that you put a small sticker on your skin, and apply sunscreen normally and over the sticker. Then when the sticker on you turns purple, you know that you have to reapply.

[Cider9986]

That's cool, do you have one?