No, the sun does not deplete CYA. The only way to lower is drain/fill or let your pool turn into a swamp over the winter...and even for some people the swamp thing doesn't change their CYA levels...CYA is a strange creature...

Oh, there is a chemical you can add to reduce the CYA, but it's very expensive and it takes a lot to do it...it's way cheaper to drain/fill.