Insomnia. Boy, oh boy, did I ever have insomnia! This was my earliest and most intense symptom. I was 15 months into my second Mirena. I never put it together until recently, but I had insomnia with the old one too. I think it completely contributes to the foggy feeling. Going to work Friday morning, I actually said aloud, "Wow, it's like I've been living in a fog."
I am 8 days post-removal, and I have slept quite well-- not perfectly, but well-- since. My husband even commented on it.