Is NADPH sensitive and unstable in UV light?
NADPH isn't destroyed by absorbing 340 nm light; if it were, then spectrophotometry wouldn't be a good technique to monitor its concentration.  The primary stability concern with NADPH and NADH is acid-catalyzed decomposition.  A great 1986 article summarizing this phenomenon is available here.

Some highlights from the article:


At 30 &deg;C, the pseudo-first order rate constant for NADPH degradation in aqueous solution goes from 0.5 min$^{-1}$ at pH ~3 to $10^{-5}$ min$^{-1}$ at pH 10.  At pH 7 the rate constant is $10^{-3}$ min$^{-1}$.
The rate is elevated slightly by higher concentrations of phosphate buffer, about 10x higher for 1 molar (!) phosphate vs. extrapolation to infinite-dilution  phosphate.


A secondary pathway for NAD(P)H decomposition is oxidation by atmospheric air, but it is usually pretty minor compared to acid degradation.

So I don't think there will be a big problem with light destroying your NADPH.  You might be able to re-use it, but it depends how long you let it sit in solution, and at what temperature.