I'm wondering if I really need to apply sunscreen even indoors.

This might sound obvious, but I've noticed there are quite a few people around me who don't apply it on cloudy days or when they're indoors. Honestly, I tend to just skim it off on lazy days too, but I end up applying it anyway because it feels a bit gross haha. I'm asking because I want to know for sure.

I've heard that UV rays are always present regardless of the weather, but I'm really curious if it's true that sitting by a window indoors affects the skin. I'm also not sure if glass actually blocks UV rays.

Also, they say to reapply every 2 to 3 hours, but I'm not sure if that's realistically possible at work or school. I'd like to know if there's a way to reapply over makeup, or if using a stick type is sufficient. I would be very grateful if anyone who knows could let me know in the comments!