All animals—from humans to birds, worms, and crocodiles—sleep. Not all species sleep alike, however, and scientists have long puzzled over which aspects are truly fundamental. Now, a new study on lizards suggests that sleep states once thought to occur only in mammals and birds have much older evolutionary origins.

Scientists had long suspected that birds and mammals are the only vertebrates to experience rapid eye movement (REM), a sleep state in which the body is mostly immobile but the brain is in overdrive. During REM sleep, the brain generates high-frequency waves of electrical activity and the eyes flicker; in humans, REM is closely linked to dreaming. Punctuating REM are interludes of slow-wave sleep, a state in which brain activity ebbs and the waves become more synchronized. This slower state is widely thought to be important to memory formation and storage.

But scientists who looked for signs of REM and slow-wave sleep in reptiles have had “confusing” results, says Gilles Laurent, a neuroscientist at the Max Planck Institute for Brain Research in Frankfurt, Germany. So when he and colleagues picked up on similar sleep patterns in Australian dragons (Pogona vitticeps) while recording their brain activity for a separate study, it came as a shock.

The team had planned to examine how the lizards—a common pet in Germany—use visual information to chase treats. They continuously recorded the lizards’ brain activity with electrodes over several weeks. At night, the sleeping reptiles’ brains produced rhythms that could be separated into two different patterns—one at very low frequency, about 4HZ, and another, higher frequency about 20HZ, the team reports today in Science. The two frequencies alternated every 40 seconds, reminding Laurent of the regular oscillations between high-frequency REM and slow-wave sleep found in mammals and birds. “The more we looked, the more it appeared as though we were looking at bona fide REM sleep,” he says.

Using an infrared camera, the team found that the sleeping lizards’ eyelids twitched during the REM-like stage, just like other animals. They also found a tantalizingly familiar pattern within the slower phase of the lizards’ brain waves. During this slow phase, electrodes picked up sharp waves of voltage, followed by ripples of electricity that closely resembled patterns seen in humans and rodents. Some scientists believe these waves and ripples help convert new information into memories by replaying past events in fast-forward. Although more studies are still needed to determine whether the function of these brain wave patterns is the same across species, the results suggest that these REM and slow-wave, sleeplike patterns could date all the way back to the common ancestor of reptiles, birds, and mammals, Laurent says.

The “provocative” findings also suggest that “there is something that goes on during sleep that is important to the function of all animals,” says Matt Wilson, a neuroscientist at the Massachusetts Institute of Technology in Cambridge. The lizards could be rehearsing the day’s events as they sleep, forming new memories of all the places they found a snack. Or maybe they’re simply dragons dreaming.

(Video credit: AAAS/Science)