I've been on the Fentanyl patch, and it did NOT impair me at all. And it certainly doesn't put you to sleep.

Now I do have to add that I am difficult to medicate, so how YOU are effected by it is unknown.

But I would SERIOUSLY doubt that you'd go to sleep!!!
They are systemic, in that the drug is absorbed into the blood stream. So you get pain relief in different areas which is good. You HAVE to put the patch either on your arm, or on your chest -- those are the instructions from the manufacturer. Evidently that is where it is better absorbed -- I'm not sure. But you are not supposed to put it anywhere else. You leave them on anywhere from 48 to 72 hours, depending on what your doctor prescribes. You can order covers for them too that help keep them on, so you can shower with them. They SAY that they stay on thru showers, but nahhh -- get the covers from the manufacturer -- they're free.
My doc usually has me have an MRI every year! But I have the spine from #$#@$ too. LOL I do think next time you should advocate for yourself and insist on the contrast material. Best of luck! Hugs, Lee