When we dream, it happens during the period of sleep known as REM sleep, and previously it has been thought that only mammals and birds were able to reach that level of sleep.  New research by a German laboratory is showing that lizards, in particular, Australian bearded dragons, may also be able to enter the dream phase of sleep, and this may bring about a new assessment of the evolution of sleep.

An article on Reuters is reporting the researchers have documented that reptiles experience rapid-eye movement, as do humans, and also enter another phase of sleep, known as slow-wave sleep.

The discussion going forward will be more about when REM and slow-wave sleep first evolved, something that scientists have been debating for a number of years.  With this new discovery, speculation that sleep traits may have begun much earlier on the evolutionary ladder will be considered, even the possibility that dinosaurs may have dreamed as well.

Certainly, it would seem that since the reptiles also experience dream sleep, the process would date back the the common ancestors of reptiles, mammals and birds.

For the research, probes were places inside the brains of five of the lizards to measure the brain activity during sleep.  Amazingly, the reptiles averaged 350 80-second REM or slow-wave sleep cycles per night.  Humans normally experience only an average of four or five.

The scientists also noted the signs of sleep stages were found in a more primitive region of the brain in the lizards, the dorsal ventricular region.  These signs are normally seen in the hippocampus in humans and other mammals.  The findings seem to discount the earlier theory that REM and slow-wave sleep was linked to warm-blooded animals, and birds and mammals evolved the process independently.

Neuroscientist Gilles Laurent, director of the Max Planck Institute for Brain Research in Germany said the research suggest these sleep traits likely evolved from a common ancestor some 300-320 million years ago.

The findings from the research were published in the journal Science.