The phrase to sleep like a top means to sleep very soundly.

In A Concise Dictionary of Phrase and Fable (1993), B. A. Phythian explains that, unlikely as it may seem, the top referred to here is the child’s toy which seems not to be moving when it is spinning, though it wobbles when being set in motion or when running down. In Glossary of Northamptonshire words and phrases (1854), the philologist Anne Elizabeth Baker (1786-1861) wrote:

A top sleeps when it moves with such velocity, and spins so smoothly, that its motion is imperceptible.

It is this period of apparent stillness (accompanied by a quiet and steady sound?) that gave rise to the phrase, which is first recorded in The Two Noble Kinsmen (1613-14), by the English playwrights William Shakespeare (1564-1616) and John Fletcher (1579-1625); the jailer’s daughter says:

(1634 edition)
O for a pricke now like a Nightingale*, to put my breast
Against. I shall sleepe like a Top else.

(* The nightingale’s song can be heard all night long and it was a common folk belief that it had to lean against a thorn so that the pain would keep it awake and singing.)

In The Old Batchelour, a comedy (1693), the English playwright and poet William Congreve (1670-1729) played on the literal and figurative meanings of the phrase:

– Is not he a Draggon that watches those Golden Pippins?
– Hang him, no, he a Draggon! if he be ’tis a very peacefull one, I can ensure his Anger dormant; or should he seem to rouse, ’tis but well lashing him, and he will sleep like a Top.

HISTORY

According to the New English Dictionary (1926), as the Oxford English Dictionary was known, a top is a toy of various shapes, cylindrical, obconic, etc., but always of circular section, with a point on which it is made to spin, usually by the sudden pulling of a string wound round it; the common whip-, or whipping-, top is kept spinning by lashing it with a whip.

Other tops, as the peg-top, are spun in the same way, but not whipped; some are spun by the action of a spring. The humming-top is a hollow top, usually of metal, with perforations, which makes a humming noise in spinning. The parish top, or town top, was a large top kept for public use, which two players or parties whipped in opposite directions.

In its 1911 edition, The Encyclopædia Britannica had added:

Other kinds of tops are made as supports for coloured disks which on revolving show a kaleidoscopic variation of patterns. The top is also used in certain games of chance, when it is generally known as a “teetotum.”

IN FRENCH

The following definitions are from A Dictionarie of the French and English Tongues (1611), by Randle Cotgrave; both gig and nun are synonyms of top:

– Toupie: feminine. A gig, or casting top.
– Sabot: masculine. A Top, Gig, or Nunne to whip, or play with; also, a pattin, or slipper of wood; also a horses hoofe.

The most common equivalent of top is the feminine noun toupie (of obscure origin, these English and French words are apparently related). Another word, the masculine noun sabot, means a clog, a hoof, but also a top, hence the phrase dormir comme un sabot, to sleep like a top.