Lizards sleep in a series of stages, such as rapid-eye movement (REM) sleep, much like human beings, according to new research. The study even indicated the creatures may dream.
Australian bearded dragons were examined as part of the new study. Investigators were able to show the animals experienced sleep in much the same way as our own species.
This marks the first time any major study has documented proof that lizards sleep in stages, including slow-wave sleep and REM. It is during this latter stage of sleep that most dreaming takes place. Before this recent study, only mammals and birds were shown to exhibit such behavior.
The subjects of a reptile's dream remains a mystery, although the possibilities remain intriguing.
"If you forced me to speculate and to use a loose definition of dreaming, I'd speculate that those dreams are about recent notable events: insects, maybe a place where there are good insects, an aggressive male in the next terrarium, et cetera. If I were an Australian dragon living in Frankfurt, I'd be dreaming of a warm day in the sun," said Gilles Laurent, a neuroscientist from the Max Planck Institute for Brain Research in Germany.
While REM sleep is marked by increased heart rate, blood pressure and dreaming, slow-wave sleep is just the opposite. During this most-restful period of sleep, little dreaming occurs, and brain activity, dominated by delta waves, is reduced.
Humans typically experience REM sleep four to five times each night. Electrodes placed inside the brains of the lizards revealed Pogona vitticeps experienced the effect around 350 times during each period of sleep. These stages lasted an average of just 80 seconds each time.
This discovery suggests that sleep patterns seen in humans and birds evolved 100 million years earlier than previously believed. Researchers believe the behavior likely first evolved in amniotes, a distant common ancestor of lizards, birds, and mammals, which lived between 300 million and 320 million years before our own time.
Nearly all animals sleep, although methods and behaviors during the process differ between species. Animals that do not sleep include dolphins and bullfrogs.
Analysis of the sleep patterns of lizards, and what the discovery can tell us about the evolution of animals, was published in the journal Science.