Certain woods will darken when exposed to sunlight, and it's usually not noticeable if it's even. However, when you have a situation like this, the protected parts have not gotten the same exposure. Exposing the whole side to the sun may even things out a bit over a long period of time, but it may never be uniform again. For one thing, the wood that has been exposed all along may slowly continue to age in tone, and the lighter stuff may never "catch up."

I won't go into the details of what would be involved in refinishing these, but you would need to use an opaque stain to mask the unevenness in tone because it's almost certainly the wood that has darkened, not the finish itself. If it were me, I'd probably just leave it alone.

I've had experience with some wood finishes, like varnish, ambering with sun exposure. So it's possible that the shadow is in the finish. But I agree with Kudzu9 that it's unlikely that further exposure will give you an even "tan" on the piece. If you enjoy refinishing furntiture, it would be worth a try. As Kudzu9 says, you may need to try to "even out" the tones with different color stains. I've had some luck with making color more uniform by simply applying one color stain more heavily to some areas and going lighter on others. Good luck.