Yes, they will lose potency. If left in open air and bright light, expect to lose it fast.
In my limited experience, oxidized spice leaves nothing to be happy about. I experienced a heavy, uncomfortable weight in my head, no visuals, and a definite need to say "aww, bummer dude."
I have heard of a way to "fix" oxidized spice, but I haven't a clue on whether or not it's myth or fact. Hope that helps.