I usually wear clear cycling glasses ($10 or $15 from Nashbar or Performance--don't recall where I got them) on "normal" days, to protect my eyes from bugs and junk. I like clear because I like cars to be able to see my eyes at intersections. These glasses are suppose to protect a bit against UV, though that's not why I wear them.

On really bright days, I sometimes wear sunglasses, so my eyes don't don't get strained from squinting.

As I understand it, UV protection is vital for sunglasses, because sunglasses without proper UV protection leave your eyes vulnerable to sun damage. Your pupils are wide open because they're behind dark lenses, which lets damaging UV rays into your eyes unless the glasses protect aginst them. But for clear lenses, your pupil will tend to adjust itself more appropriately for the actual sunlight (and UV). An eye expert may want to put a finer point on this, but that;s the general idea.

I don't wear protective glasses (clear or shaded) religiously. I forget them, or I just don't feel like wearing them. And they're a bit less necessary since I out full fenders on my bike, which protects my face from most flying crud (aside from bugs).