Quoted from ForceFlow:

Sunlight is sunlight. You still would have UV rays bouncing around just like the light you see.
If you want to protect a game from fade, either block the light from coming in, or cover the game(s) in a material that doesn't let light through.

Quoted from SuperDaveOsbourn:

Anything that causes the breakdown of material will change/alter/degrade the form of light that is absorbed, and thus seen by you. In other words, yes indirect light will cause fading. I have seen it on old pins on location that were protected (black sheets on the windows), yet doors nearby were left open in the hot months and light hit, bounced, and reflected onto the pinball machines. Not much you can do about it, other than put your stuff into a oxygen (that also causes aging, via oxidation) free environment, that is free from gravity, and also never touched by human hands. Oh, and about light and how you view/see things in life... Read this book.
Health and Light: The extraordinary Study that Shows How light Affects Your Health and emotional well being.
You may change your views on how to deal with light, appreciate its power on your well being, and emotional views on pinball, pinball people, and of course why you are spending so much time worrying about light on your games, and not so much time on how it may be a good thing for you and your pinballs