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?
Best Answer
The phrase sleep like a top appears in The Two Noble Kinsmen by John Fletcher and William Shakespeare, which was first performed in 1613–14, and published in 1634.
There is a possible clue to the etymology in The Countesse of Pembrokes Arcadia, Volume 2 (somewhere between 1580 and the author's death in 1586) By Sir Philip Sidney, in which the phrase like a toppe is used to express the stationary nature of a top, which can only be moved by whipping it.
From this, I believe it might be possible that it is the stationary nature of a top, which requires whipping to move, or wake, it that gives rise to the phrase sleep like a top.