Crib Chewing

Any ideas on how to make a little girl stop sharpening her teeth on her crib? A few days ago I noticed some teeth marks on Sarah's crib and since then it's just been getting worse. Yesterday I caught her in the act of chewing and sharpening her teeth on her crib. I'm not sure when she even does this. I think it's when she first gets up in the morning and from her naps. Who knows how long she's even up before I hear her little sounds in the morning. I usually find her sitting in her crib happily chatting and playing with her talking Pooh bear.

Oh well, at least we bought the crib used and didn't spend much money on it. The two railings have protectors on them, so they're fine. The part that she's chewing at is the end board.