Lizards sleep in stages, just like humans, other mammals, and birds, indicating that the phenomenon goes back to a common ancestor that may have existed over 300 million years ago.
Discovery News reported that slumber characterized by stages of rapid eye movement (REM) and slow-wave sleep (SWS) occurs in the bearded dragon species that is native to Australia and not uncommon as a pet.
Previously, scientists had placed electrodes on the lizard's heads, however a new study published in the journal Science saw researchers placing probes directly into the brains of five bearded dragons.
"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 study 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."
Humans' SWS-REM cycles typically last from 60-90 minutes, and we usually get four or five cycles in during a good night's rest.
Lizards, on the other hand, appeared to go through roughly 350 cycles that lasted 80 seconds each.
This disparity in cycle length was generally consistent with previous findings that suggest the length of an animal's sleep cycle is somewhat linked to its size.
"For example, the human sleep cycle is about 60–90 minutes, while it is 30 minutes for cats and 15 minutes for rats," said Laurent.
Beyond that, smaller creatures like insects and arachnids sleep, but it is unclear if they experience SWS-REM cycles. Some animals, such as the jellyfish, appear to rest but not sleep, as they do not have a brain like vertebrates do.
Matthew Wilson, a professor of neuroscience at M.I.T. who has studied sleep,
told The New York Times that the findings of the study could generate some controversy.
"Like any good science, it raises more questions than it answers," said Wilson.
"It forces us to think about the earliest evolution of these phenomenon. When did these aspects of sleep start, and what were they for?" asked another professor, Daniel Margoliash, a professor of organismal biology and anatomy at the University of Chicago.