Why/how would sunlight harm a glow disc? It does seem counter-intuitive, and I haven't heard this before.
Well, now you've got me. I don't know. In fact, what I said earlier may be simple bullsh*t. And, after 20 mins of google searching, I still don't know. One thing I do know is that I have some older (1-2 yrs) glow discs that don't glow any more. I must have thought that the phosphorescent powders that they use in glow plastic just wear out somehow.
I did learn something about the quantum mechanics behind phosphorescence ... all about forbidden energy state transitions, spin multiplicity, triplet states, and quantum yield ... ie., why discs glow and why some glow longer than others. However, I still don't know why mine stopped glowing and whether it has anything to do with light or not.