According to electroencephalograph recordings of sleep states in animals, it is possible to believe that some animals dream.

REM sleep, the "dreaming sleep," has been reported in most studied mammals, as for instance, rats, cats, monkeys, opossums, elephants, and also in birds (Encyclopedia Britannica, 307).

Among mammals, only the echidna (an animal similar to porcupine) does not present REM sleep (Allison et al., 1972, BN 615).

Because REM sleep is recognizable in mammals and birds but not in snakes and other reptiles, Jouvet (ref. ?) suggested that REM [sleep] may be a later phylogenetic process related to warm-blooded animals.

REM sleep appears about once in every 90 minutes in human beings, and about once in every 25 minutes in cats. Each episode lasts for several minutes.

Comparative percentages of total waking
and rapid eyes movement (REM) sleep in some animals (Based on Rojas-Ramirez and Drucker-Colin, 1977 BN, 615)

Why Do Some Animals Sleep so Little?

The amount of active sleep is inversely related to the degree to which the species must cope with the predatory danger. Species vulnerable to predators tend to have little active sleep (BN, 615 - Allison and Circhett, 1976). This finding is not surprising, since animals are minimally responsive to external stimuli during active sleep. The fact that large animals tend to have little sleep is understandable, since the larger animals studied (cow, horse, donkey, elephant) must spend large amounts of time foraging.

Because REM sleep is recognizable in mamals and birds but not in snakes and other reptiles, Jouvet suggested that REM may be a later phylogenetic process related tp warm-blood animals.

REM sleep comes about once every 90 minutes in human beings and about once every 25 minutes in cats. Each episode lasts for several minutes.

Author: Silvia Helena Cardoso, PhD. Psychobiologist, master and doctor in Sciences by the University of São Paulo and post doctoral fellowship by the University of California, Los Angeles. Invited Professor and Associate Researcher of the Center for Biomedical Inofrmatics, State University of Campinas (Unicamp), Brazil.