Owners of pet lizards for years have anecdotally reported that their reptilian friends snore, appear to dream, and seem to enjoy snoozing just as we do, and now new research finds that reptiles do indeed experience the same sleep states of mammals and birds.
Before, it was thought that only birds, humans and other mammals went through rapid eye movement sleep (REM) and slow-wave sleep (SWS) as they snoozed, but the new findings, published in the journal Science, suggest that these sleep patterns evolved in the common ancestor of all such animals more than 300 million years ago.
By extension, it is then likely that today's reptiles dream, and that many long-extinct animals such as dinosaurs did so as well.
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"Dreaming, like sleep, consciousness, language, pain etc. are all concepts or phenomena that were first experienced and described in a self-referential manner by humans, assigned a word and implicitly considered as uniquely human," senior author Gilles Laurent, director of the Max Planck Institute for Brain Research, told Discovery News.
"But if you think as a biologist about these phenomena, accept that most of them probably did not drop from the sky onto the first human being, but rather result from some slow evolutionary process, then we can start thinking about dreams as patterns of neuronal activity in the brain during sleep that are at least partly built up from past experience," he added. "If you are ready to accept that bits of neuronal playback in certain brain areas during sleep can be called dreams, then I'll bet that lizards dream."
Scientists for decades tried to detect sleep patterns in reptiles by placing electrodes on the surface of the animals' brains, similar to doing an EEG. Laurent and his team took this a step further, by implanting probes right into the brains of five bearded dragons (lizards). As a result, the researchers were better able to record the sleep patterns and to match them to eye movements.
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Humans go through four or five 60 to 90 minute SWS-REM cycles on average every night, but the lizards experienced about 350 80-second cycles. Rest is unaffected, Laurent said, as the two sleep cycles run together, just as they do in humans. The individual is basically unaware that all of this brain wave change is happening.
"Among mammals, there tends to be some relationship between animal size and length of sleep cycle," he said. "It is not absolute, but is statistically true. For example, the human sleep cycle is about 60–90 minutes, while it is 30 minutes for cats and 15 minutes for rats."
Even insects and arachnids like spiders sleep, but it is unclear if they experience REM and SWS sleep patterns. Some animals rest, but do not technically sleep.
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Laurent explained, "Multicellular animals without brains, such as sponges and jellyfish...most likely do not express 'brain sleep' both by definition since they lack a brain and because REM and SWS require certain neural circuits that, as far as we know, they lack."
Even for these creatures, prior amounts of rest, temperature shifts and other environmental factors seem to affect rest, which in multicellular animals without brains probably is expressed through their circadian rhythms, otherwise known as their internal "body clocks."
Snoring Lizards Reveal Sleep Predates Dinosaurs: Page 2