A couple of months ago I upgraded to Office 2013 at work. I liked it, but around the same time, my eyes started burning. I never made the connection, but then last week, when a coworker upgraded, he mentioned his eyes were burning, and he made the connection.
He found this guide for toning down Office. We both recommend the dark gray scheme, which is much easier on the eyes than the default harsh white scheme.
The other thing is for whatever reason the brightness and contrast on my monitors was cranked up to 75. I toned them both down below 50. That was an old trick for extending the useful life of aging CRTs, but these LCD monitors were still readable with brightness and contrast set in the 40s.
The changes beat wearing sunglasses indoors, which, believe me, I had considered.
I also found I had visual artifacts, like corrupted windows, on my Lenovo L440, which uses Intel integrated graphics. Turning off hardware acceleration solves that. Normally I wouldn’t want to do that, but Office isn’t terribly graphics intensive, and a fast unreadable display is still unreadable. That setting is in File -> Options -> Advanced -> Display -> Disable Hardware Acceleration.
With those items fixed, I found Office 2013 to be much more usable.