Scientists have long suspected that the stage of REM sleep (during which there is a rapid movement of the eyeballs) is peculiar among vertebrates, not just birds and mammals.

During rapid sleep, the brain produces high-frequency electric wave, and the person sees dreams. He alternates with slow-wave sleep, when brain activity falls, but intense memories are formed.

Reptiles such cycles never seen before. However, in the study of brain activity Agam bearded (Pogona vitticeps) are popular Pets in Germany — scientists unexpectedly found during sleep Agam rhythms in two types — low frequency (four Hertz) and high frequency (20 Hz). They alternated every 20 seconds, reminding biologists that similar developments of rhythms in the sleep of mammals.

Then, with the help of infrared cameras, biologists have discovered that the lizards eyelids tremble as well as during REM sleep in mammals. Finally, during slow sleep, the electrodes recorded from Agam pointed pulsating waves in humans and rodents, they probably help to keep events in memory, "scroll" them into fast-mode.

Thus, two-phase sleep (and dreams) could arise even in the common ancestor of reptiles, birds and mammals, the biologists.