'Concerning the sleep and watchfulness of animals: - It is quite manifest that all viviparous animals with feet both sleep and are awake; for all that have eyelids sleep with the eyes closed; and not only men appear to dream, but horses, oxen, sheep, goats, dogs, and all viviparous quadrupeds. Dogs show this by barking in their sleep. It is not quite clear whether oviparous animals dream, but it is quite plain that they sleep.

'Man sleeps the most of all animals. Infants and young children do not dream at all, but dreaming begins in most at about four or five years old. There have been men and women who have never dreamt at all; sometimes such persons, when they have advanced in age, begin to dream; this has preceded a change in their body, either for death or infirmity.' - History of Animals.