Nap (n) - A short sleep. Believe it or not, this word is one of the old ones in English. Chaucer used it way back in the 14th century. From the Old English word nappen. And I just love this, it is perhaps from an earlier Old Norse word, hnipna, meaning to drool.
I suppose, truly, that the master of the house has kept you so busy in bed all night long that now you need a nap. ~ Geoffrey Chaucer, The Shipman's Tale