Excerpted from "What Do Dogs Know?":

The amount of sleep that an animal needs depends upon its species. Horses and cows may sleep only three or four hours daily, while bats and opossums may sleep closer to 20 hours. The various breeds of dogs also seem to have different sleep requirements.

Some very large breeds of dogs, like Newfoundlands, Saint Bernards, and mastiffs, often spend a great deal of their lives sleeping---perhaps up to sixteen or even eighteen hours a day. For this reason they were often referred to as "mat dogs," because they could always be found lying in Front of the fireplace (much like a giant, furry hearth mat). If it is true that dogs dream about what they do, then we must conclude that Great Danes probably only dream about sleeping.