from "What Do Dogs Know?":
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.