I have slept really well since I had my Mirena, unless it is coincidence or the oestrogen that I started at the same time, but I have never felt sedated.
That doesn't answer your question though, but I did not find utrogestan sedating at all...I felt wired up to the electricity supply, especially at night
So maybe the progesterone in the Mirena is firing you up! We all seem to tick in different ways.