An explanation for the English expression "sleeping like (or as sound as) a top" is here. Apparently case closed. It derives from the Italian expression Ei dorme come un topo with topo being wrongly translated as top rather than mouse.
However mice aren't known for sleeping, only dormice are, and the Italian for dormouse appears to be ghiro. The expression Dormire come un ghiro (To sleep like a dormouse) also seems common. The root giro/gyro in English indicates spinning (gyroscope, gyrocopter, autogiro). Is this just a meaningless coincidence? That two words for mouse in Italian (topo and ghiro) are both cognate with spinning things in English? Or is ghiro involved in some way in the mistranslation?