Do you think its dogs? Dogs sleep a lot. Guess again. What about cats? Not them either. It isn't a primate, so that rules out humans, apes, and monkeys.

Sleep researchers divide the phases of sleep into two broad categories: REM and NREM. In REM (rapid eye movement) sleep, your muscles are almost completely paralyzed, but your brain's activity is nearly indistinguishable from the activity of an awake brain. But while you're in REM sleep you're nearly impervious to the goings-on around you. In NREM (non-REM) sleep, the level of muscle paralysis is lower, brain activity is reduced, and you're somewhat more aware of environmental stimuli. It's thought that REM is the part of sleep where the most dreaming occurs, and it's also the part of sleep most commonly associated with memory consolidation and learning.

The species that gets the most REM sleep on our planet? The duck-billed platypus.

Image: John Gould/Public domain (source)

According to a study led by UCLA psychiatrist J. M. Siegel published in the journal Neuroscience in 1999, the Australian and Tasmanian critters get up to 8 hours of REM sleep a day. Our own species gets a paltry two hours or so of dream-filled REM each night.

Platypuses are unique in that they're part of a group of mammals called "monotremes," which diverged from the rest of mammals very early in the evolution of the mammalian line. Because their REM is so similar to our own, Siegel's findings "suggest that the immediate reptilian ancestors of the early mammals either had REM sleep or had a state with many of the neural correlates of REM sleep, or that REM sleep evolved very rapidly in the mammalian line."

Since birds also have REM sleep, it might tell us something about dino dreams. "Since REM sleep is present in birds, the most parsimonious hypothesis is that REM sleep evolved only once and was present in the common ancestors of birds and mammals. If this is the case, the dinosaurs ancestral to the birds may also have had REM sleep," he says.

And if they had REM sleep, it is at least possible that they dreamed. What might theropod dinosaurs have dreamed about?

[Neuroscience; NIH]