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 Scientists said on Thursday they have documented for the first
time that reptiles, like people, experience rapid eye movement, or
REM, sleep and another sleep stage called slow-wave sleep. Until
now, only mammals and birds were known to experience these.
Because REM sleep is when dreaming occurs in people, the findings
suggest that these lizards dream, too. But, what would bearded
dragons dream about?
"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," said
neuroscientist Gilles Laurent, director of the Max Planck Institute
for Brain Research in Germany.
"If I were an Australian dragon living in Frankfurt, I'd be dreaming
of a warm day in the sun."
 When REM and slow-wave sleep first evolved has remained a mystery.
The discovery that reptiles share these important sleep stages with
mammals and birds, sister groups among the land vertebrates,
suggests the sleep traits emerged far earlier than previously
suspected in the common evolutionary ancestors of the three groups.
In human REM sleep, the eyes move rapidly, the heart rate and blood
pressure rise, limb muscles become incapacitated and dreams
flourish. Slow-wave sleep, also called deep sleep, is the most
restful sleep period, marked by slow brainwaves and little dreaming.
These stages are thought to be useful for consolidation, storage and
erasure of memories and other purposes.
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The researchers placed probes inside the brains of five bearded
dragons, which can reach 24 inches (60 cm) in length, to measure
electrophysiological activity during sleep. While people experience
four or five long slow wave/REM sleep cycles nightly, the lizards
averaged 350 80-second-long cycles.
Some of the telltale signs of these sleep stages, seen in the
brain's hippocampus in mammals, were found in a more primitive brain
region, the dorsal ventricular ridge, in the lizards.
Some scientists had hypothesized REM and slow-wave sleep might be
linked to warm-bloodedness and evolved independently in birds and
mammals.
Laurent said the findings suggest these sleep traits probably
evolved at least as long ago as in the common ancestor of reptiles,
birds and mammals: small lizard-like animals that lived between 300
million and 320 million years ago.
The research was published in the journal Science.
(Reporting by Will Dunham; Editing by Bill Trott)
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