A new study suggests that rapid eye movement (REM) and slow-wave sleep (SWS) may even have occurred in dinosaurs.

The study conducted by German scientists has found that lizards and other reptiles undergo slow-wave sleep as well just like birds, humans and other mammals. It said that REM and SWS could have evolved with the first amniotes that lived 300 million years ago, and are the common ancestor of mammals, birds and reptiles.

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In finding the result, researchers implanted a type of silicon probe in the forebrain regions of five Australian bearded dragons in order to monitor brain activity as the reptiles slept. They also monitored the lizards' eye movement as they slept, using infrared cameras and computerized video analysis.

“[The study provided] extremely strong evidence that the patterns of structure of sleep that we've seen in a broad range of species is reflective of something that evolved very early in vertebrate evolution and is shared across many - perhaps all - vertebrates,” Dr. Daniel Margoliash, a professor of organismal biology and anatomy at the University of Chicago who was not involved in the study.

Laurent and the team found differences between the sleep of lizards and mammals, however. In humans, for example, the REM phase of the sleep cycle is relatively short. A person might experience just 5 minutes per hour of slow-wave sleep at the beginning of the night, and as this phase gradually lengthens, 15 or 20 minutes per 90 minutes of slow-wave sleep later on. In the Australian dragon, the ratio of REM to slow-wave sleep is about 50-50. The cycles happen between the two every minute or minute and a half. They experience up to 350 sleep cycles per night, compared to four or five in humans.

REM sleep is characterized by brain waves that look similar to waking brain activity. In mammals, the large muscles of the body are immobile, but the eyes twitch randomly during REM sleep. People do most of their dreaming in this phase of sleep. Slow-wave sleep is a non-REM sleep phase that is marked by slow brain waves called delta waves.

TagsDinosaur lizard, dinosaur, lizard, australian dragons, rapid eye movement, slow-wave sleep, Mammals, reptiles