Hmmm... Odd. I was prescribed adderall for day time insomnia and I think it helps. The only reason I could think of that it would cause sleepiness is that a side effect of cymbalta is that it can cause daytime insomnia. If it's being prescribed as a "booster" drug for cymbalta it is most likely amplifying it. Technically doing it's job for which it was given to you. You may want to tell you doctor about this and ask if it can be lowered a little bit to reduce the side effects while keeping it doing the same thing.

I take adderall 10mg 2x/day "as needed" because I have sleep apnea side effects mostly to help me stay awake during school. I have been taking it for a few months and I'm not sure if it's really working.