Do Spiders, Cuttlefish, Beared Dragons Dream?

Rößler isn’t the only researcher thinking about such questions in animals distantly removed from ourselves. Today, researchers are finding signs of REM sleep in a broader array of animals than ever before: in spiders, lizards, cuttlefish, zebrafish. The growing tally has some researchers wondering whether dreaming, a state once thought to be limited to human beings, is far more widespread than once thought.

REM sleep is generally characterized by a suite of features in addition to rapid eye movements: the temporary paralysis of skeletal muscles, periodic body twitches, and increases in brain activity, breathing and heart rate. Observed in sleeping infants in 1953, REM was soon identified in other mammals such as cats, mice, horses, sheep, opossums and armadillos.

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Researchers have since observed a similar state in octopuses. If octopuses and cuttlefish dream, “it just kind of blows down the walls of what we think about humanity being so special,” Iglesias says.

Researchers have also observed a REM-like stage in bearded dragons by recording signals from electrodes in their brains. And they have reported at least two sleep states in zebrafish based on the fishes’ brain signatures. In one of the states, neural activity synced up like it does in a non-REM stage of mammals. In another state, the fish showed neural activity reminiscent of a waking state, as happens in REM. (The fish didn’t show rapid eye movements.)

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With all these signs, it’s fair to posit that animals could be dreaming, Ungurean says. “However, if we take these reasons one by one, it turns out that none of them is sufficient.” The brain activity associated with replay, like that of the maze-running mice, doesn’t occur only during REM or sleeping, Ungurean says. It can also occur during planning or daydreaming. And the link between REM and dreaming isn’t absolute: Humans dream in non-REM too, and when drugs are used to suppress REM sleep, human study participants can still have lengthy and bizarre dreams.

Ultimately, people know they are dreaming because they can report it, Ungurean says. “But animals cannot report, and this is the biggest problem that we have in purely scientifically and robustly establishing this.”

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