Here's my experience.
I flew the North Pac for years between ANC and Asia. The sun was on my left arm for hours headed westbound and I began to notice little lesions (not really the technical term) on my arm. After wearing a long-sleeve cruise shirt, the lesions began to disappear or reduce in appearance. The left sleeve of the shirt also noticeable faded.
After some research on this, I found that the cockpit windows do not block any UV. My ophthalmologist suggested that I get a UV block coating on my sunglasses, which I did.
Scoff if you will, but there is something to this UV concern.