Sleep and Dreams in the Animal Kingdom

With few exceptions, all animals, including all invertebrates and vertebrates, experience periods of restfulness or inactivity. But only some lineages really sleep, meaning that they enter a substantially altered state of brain activity, triggered by internal factors, not just by a need to stop moving. Under this definition, sleep appears to occur in some invertebrates in the arthropod lineage, such as flies and crabs. But it is much more widespread in vertebrates. The first vertebrate deep sleepers probably lived more than 250 million years ago.

All amniotes (animals that develop in an amniotic sac) including all reptiles, birds, and mammals alive today experience deep sleep. This state is easily detected with EEG as an extended period of slowed electrical potentials called slow-wave sleep (SWS). REM sleep, which follows long periods of SWS in a sleep cycle, seems to have evolved later. REM is found in all mammals and all birds, but evidence of REM in other reptilians has historically been mixed. However, recent EEG recordings in Australian dragons (a kind of lizard) found evidence for some degree of REM, suggesting that an ancestor shared by all reptiles, birds, and us, was the first to experience REM.

The Content of Mammal Dreams

One implication of the universality of dreaming among mammals is that we can combine this knowledge with what we know about mammals' sensory experience during wakefulness. Whatever it feels like to “see,” "hear," “understand,” or “be aware of” something for a cat should be approximated in their dreams, albeit with some abstraction. At the same time, we know that mammals vary considerably in the relative power and precision of their sensory systems. Whatever the perceptual content of an animal's dreams, we should expect that the progression of experiences in all animals' dreams is just as chaotic (and, at times, repetitive) as it is in our own dreams. If this is the case, we can only wonder how they interpret these strange experiences upon waking. Maybe they are just as confused and fascinated as we are.

Reflection: What do you think animals dream about?

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#Dreams #Animals #REM-Sleep