To sleep like a top is to sleep very soundly.

Similes of the ‘like a’ type usually relate some verb with a noun that possesses the property the simile is aiming to convey; for example, ‘drink like a fish’, ‘go over like a lead balloon’. Why ‘top’, or why ‘log’ in the related term ‘sleep like a log’ for that matter?

sleep like a topTops, or more correctly spinning-tops’, were popular amusements in the days before children had access to toys requiring batteries. The British Museum has on display tops from Egypt, dating from around 1250 BC. When a top is spinning well the precessional effect causes its axis to remain stationary and it can appear to be still, that is, ‘sleeping’.

The expression ‘sleep like a top’ is quite old. The earliest citation I know of is the 1634 edition of Fletcher and Shakespeare’s play The Two Noble Kinsmen:

“O for a pricke now like a nightingale, to put my breast against. I shall sleep like a top else.”