stress doesn't cause acne. not washing your dirty face because you're too stressed out does. so it's basically because your glands produce more sebum, no matter on what emotional condition you are.
the sun won't conspire with you to beat acne. it's just make your skin looks more tanned and your acne looks more less severe. nothing more than a visual trick. it can make it worse, though, since being exposed to the sun make your glands produce more sebum.