Scientific American 201905

May 2019, ScientificAmerican.com 25

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SLEEP SCIENCE

Cuttlefish

Dreams

The marine invertebrates

appear to experience

rapid eye movement sleep

Cuttlefish are known for their sophisti­

cated camouflage, as well as their kaleido­

scopic displays for attracting mates and

mesmerizing prey. These close relatives of

squid and octopuses achieve such feats

via millions of chromatophores—tiny sacs

of pigment under the skin attached to mus­

cles that squeeze or relax to push colors to

the surface. In a new study, researchers

report they have observed resting cuttle­

fish cyclically changing color and twitching

their limbs in phases that resemble rapid

eye movement (REM) sleep.

Many animals experience REM, a sleep

phase that may involve eye movements and

loss of muscle tone; in humans and some

other mammals, it is often linked to dream­

ing. Study co-author Teresa Iglesias, a biolo­

gist at the Okinawa Institute of Science and

Technology, cannot yet say whether cuttle­

fish experience the same kind of sleep that

we do, but the animals’ active phase during

rest is similar to the REM sleep observed in

humans and other vertebrates. Although

the lineage that includes cuttlefish diverged

from vertebrates around 500 million years

ago, the study findings hint at a common

evolutionary origin for sleep, she says.

Iglesias and her colleagues filmed cut­

tlefish in laboratory tanks for 24 to 48

hours at a time. While resting, the animals

would demonstrate REM-like behavior for

periods lasting between two and three

minutes. They made sporadic arm and eye

movements, and the chromatophores

around their eyes got darker. Such phe­

nomena were not observed during waking

activity or inactive parts of their “sleep”

cycles. The findings were published in Jan­

uary in the Journal of Experimental Biology.

“Sleep is a neural phenomenon more

than a behavioral one,” says psychologist

Jennifer Mather, a cephalopod expert at the

University of Lethbridge in Alberta, who

was not involved in the new research.

Cycles of brain activity while an animal is

quiescent, she explains, suggest sleep may

play a role in stabilizing neural circuitry and

processing newly acquired information. The

REM-like behavior observed in cuttlefish

could indicate a similar process is happening

in their brain, she says.

But do cuttlefish dream? “We can specu­

late all we like,” Mather says. “It’s difficult be -

cause I don’t [even] think we know why we

dream.” Iglesias says it is not impossible, but

there is not enough evidence to answer this

question. “For now,” she says, “the biggest

takeaway is that we need to dig deeper and

keep an open mind.” — Jim Daley

© 2019 Scientific American

May 2019, ScientificAmerican.com 25

GETTY IMAGES

SLEEP SCIENCE

Cuttlefish

Dreams

The marine invertebrates

appear to experience

rapid eye movement sleep

Cuttlefish are known for their sophisti­

cated camouflage, as well as their kaleido­

scopic displays for attracting mates and

mesmerizing prey. These close relatives of

squid and octopuses achieve such feats

via millions of chromatophores—tiny sacs

of pigment under the skin attached to mus­

cles that squeeze or relax to push colors to

the surface. In a new study, researchers

report they have observed resting cuttle­

fish cyclically changing color and twitching

their limbs in phases that resemble rapid

eye movement (REM) sleep.

Many animals experience REM, a sleep

phase that may involve eye movements and

loss of muscle tone; in humans and some

other mammals, it is often linked to dream­

ing. Study co-author Teresa Iglesias, a biolo­

gist at the Okinawa Institute of Science and

Technology, cannot yet say whether cuttle­

fish experience the same kind of sleep that

we do, but the animals’ active phase during

rest is similar to the REM sleep observed in

humans and other vertebrates. Although

the lineage that includes cuttlefish diverged

from vertebrates around 500 million years

ago, the study findings hint at a common

evolutionary origin for sleep, she says.

Iglesias and her colleagues filmed cut­

tlefish in laboratory tanks for 24 to 48

hours at a time. While resting, the animals

would demonstrate REM-like behavior for

periods lasting between two and three

minutes. They made sporadic arm and eye

movements, and the chromatophores

around their eyes got darker. Such phe­

nomena were not observed during waking

activity or inactive parts of their “sleep”

cycles. The findings were published in Jan­

uary in the Journal of Experimental Biology.

“Sleep is a neural phenomenon more

than a behavioral one,” says psychologist

Jennifer Mather, a cephalopod expert at the

University of Lethbridge in Alberta, who

was not involved in the new research.

Cycles of brain activity while an animal is

quiescent, she explains, suggest sleep may

play a role in stabilizing neural circuitry and

processing newly acquired information. The

REM-like behavior observed in cuttlefish

could indicate a similar process is happening

in their brain, she says.

But do cuttlefish dream? “We can specu­

late all we like,” Mather says. “It’s difficult be -

cause I don’t [even] think we know why we

dream.” Iglesias says it is not impossible, but

there is not enough evidence to answer this

question. “For now,” she says, “the biggest

takeaway is that we need to dig deeper and

keep an open mind.” — Jim Daley

sad0519Adva3p.indd 25 3/21/19 3:21 PM

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© 2019 Scientific American