Philip K. Dick's 1968 novel, "Do Androids Dream of Electric Sheep?" explores a number of philosophical issues related to the nature of being human. It became the inspiration for the influential 1982 movie, "Blade Runner." The title of the novel indicates the importance we attach to dreaming as an important characteristic of the human experience. As we move toward what some futurists such as Ray Kurtzweil call the technological singularity we may ask: Will intelligent or even spiritual machines need to sleep, to dream? While this question may still lie in the future it is also possible to ponder whether other creatures, the animals already in our world, also dream.
REM sleep is associated with episodes of vivid dreaming was and first discovered by Aserinsky & Kleitman in humans during the 1950's. Awakening people during REM sleep typically results in a report of vivid dream imagery and a complex associated story line. Dream imagery can occur in other sleep states but is typically less vivid and complex. After the REM state was identified in humans, a similar state was discovered by Dement a few years later in cats. As in humans, cats in REM sleep show a low voltage EEG with characteristic eye movements. There is also a loss of muscle tone (atonia), presumably to prevent acting out of the dream content. This striking cross species similarity of brain states and behavior indicated that dreaming may not be limited to or be a uniquely human state of consciousness. In fact, much of the initial work on REM sleep was done on cats and the underlying brain centers involved in REM sleep were discovered in the cat.
Further research quickly revealed that REM sleep was not limited to humans and cats. On the contrary it was found to be widespread among animals. With only a few exceptions REM sleep appears to be nearly universal among mammals and birds. An unusual anteater found in Australia does not show any evidence of REM-like EEG patterns. More recent investigations, however, indicate that there may be patterns of brain stem activation that are similar to those seen in REM sleep in other animals. Certain marine mammals such as dolphins have evolved unihemispheric sleep so that only one side of the brain shows evidence of deep sleep at a time. The other hemisphere remains alert allowing these creatures to keep swimming and responding to the environment. It is unclear whether or not dolphins have any REM sleep at all and if they do it is less than 15 minutes a day. Birds show REM sleep but in small amounts that occur in brief episodes that last only a few seconds at a time.
The purpose of REM sleep remains unclear but does seem related to the degree of maturity of an animal at birth. Animals that are very helpless at birth, such as ferrets and armadillos, have large amounts of REM while more mature animals at birth such as cattle and horses have relatively little. Humans are born intermediate between the most helpless and most mature animals and have moderate amounts of REM sleep at birth. It may be that REM sleep has a role in the development of the brain. For unknown reasons animals that have a lot of REM sleep at birth also continue to have a lot of REM sleep as adults.
Of course, having REM sleep does not absolutely indicate that the same subjective experience of an alternative world of dreams exists in other animals. Yet-anyone who has seen a cat or dog sleep and have some loss of the muscle atonia associated with the REM state must be struck by what looks like an effort to run and move that certainly suggests an ongoing dream.
While we still do not know exactly why we sleep and dream, it seems to have served an important role in the evolution of animal brain function. Whether or not it is somehow necessary to awareness or consciousness is not known. We will have to await either for the emergence of intelligent machines or contact with some complex extraterrestrial life to have a better answer to this question. It does appear, however, that we can answer the question of whether cats dream of catching mice - in the affirmative.