New studies reveal that more animals are dreaming than we thought. In fact,
all mammals and birds have REM, and if J.M. Siegel is correct, reptiles may have
REM as well.
REM, or Rapid Eye Movement sleep is a regularly occurring stage of sleep in
which, when people are awakened and asked, dreams are often reported. In this
stage of sleep, which occurs about six times a night for an average of twenty
minutes each, our eyes move under our eyelids as if we were awaking and scanning
some scene, hence the designation Rapid Eye Movement sleep.
After the 1953 discovery of REM in humans by modern science, researchers
began testing other species for REM and searching for signs of dreaming in all
kinds of creatures. They found that most mammals had REM, but couldn't find REM
A theory developed that dreaming was an evolutionary advance to keep the
mammalian brain warm and alert and to not let it sink too deeply into
inactivity. Oddly, some mammals seem to lack this sleep stage, particularly the
egg laying echidna and the friendly dolphin.
Now these older studies are being questioned as new research goes beyond the
old methods of just looking for eye-twitching during sleep. REM sleep has three
main areas that need to be obtained for counting as true REM. The first is the
activation of the brain, the second is the body/sensory activity (motor output
down - sensory input down), and the third is a change in the neuromodulatory
systems. Motor shut-down is experienced as partial body paralysis, called atonia.
At this time, the only studies that have been able to measure all three of these
are studies with cats, but major advances in studying the first two now allow
researchers to more closely monitor the sleep in animals. These new techniques
have lead J.M. Siegel to re-examine some of the animals who were said to not
have REM dreaming sleep.
Who's dreaming the most?
Sleep itself is somewhat different for every species. Dolphins, for example,
exhibit slow circle-swimming, where they appear to allow half their brain to
sleep at a time. Birds are known to fly continuously for days, and it is
suspected that they have some kind of sleep or partial sleep during this
behavior. But all species show some form of sleep.
J. M. Siegel reports that the early studies of Zepelin and Rechtschaffen
(1974-1994) show that the smaller the animal the more REM sleep. Of course, the
smaller the animal the more sleep in general they get. Elephants and giraffe
sleep three to five hours, while the ground squirrel and brown bat sleep
seventeen and twenty hours. REM sleep varies from forty minutes a day in cattle
to seven hours a day in some opossum.
The amount of REM doesn't seem to be about being a more evolved mammal.
Primates with higher intelligence, abilities, and lifespan have less REM than
rats. And though the time per species seems fairly fixed, the time can vary
widely within an Order, indicating that it is not directly correlated with the
evolution of that Order.
Another interesting correlation is between altricial and precocial mammals.
Altrical animals are born too immature to care for themselves (cat, rats,
humans) while precocial animals like the horse and guinea pig can. The altricial
animals tend to have more REM, though it decreases as they age. Precocial
animals have less REM but the amount doesn't vary as much over their lifetime.
Siegel (1999) sites Zeplelin and Jouvet-Mounier on this theory: "Zepelin showed
that immaturity at birth is the single best predictor of REM sleep time
Just what this means is unclear. Does REM somehow function to help undeveloped
parts of the brain? We know that REM sleep is somewhat separate from dreaming,
and that dreaming can occur in other NREM states. It may be that in the young
and neonates that REM has more to do with finishing up the job of hardwiring the
brain and less to do with dreaming. Then later, REM functions to help more with
other aspects of brains functioning.
Do All Mammals Dream?
It appears that they (Birds, Mammals) all do dream if one's definition of
dreaming and REM are loose enough. However this is a fairly new viewpoint.
When the echidna spiny anteater was first studied, they couldn't find any
evidence for REM sleep. The researchers, Truett Allison and Henry Van Twyver in
1970 concluded that the echidna had no REM sleep. This had a major impact on
theories of sleep evolution. (See Table 1) Of the three mammal groups, two
(placental and marsupial) had REM and one (Monotremes - Echinda, Platypus)
didn't have REM. This meant that REM must have evolved ~after~ these three
branches split (150-200 million years ago), but while placental and marsupials
were still together, as the possibility of REM developing independently after
the groups were separated seems less likely than a common ancestor.
The Siegel Group re-tested the echidna with more sophisticated equipment and
looked into different brain areas of the creature, finding eye-movements and
twitching, but also something else. The echidna seemed to have less strong REM
like brain wave patterns, but had these little-bursts more often. It was as if
the creature was getting micro-dreaming throughout the night.
The Siegel Group then tested another monotreme, the platypus. This was very
difficult as platypus does not do well in captivity and had never been carefully
tested before in a sleep-REM study. The platypus exhibited many REM indicators,
eye-movements, body atonia, and brain wave bursts characteristic of REM.
However, there wasn't the low voltage REM characteristic of most mammals.
Siegel speculates that these differences in these two monotremes, echidna and
platypus, have to do with the way these creatures live. The platypus dives down
into deep burrows beneath the water and can afford long, luxurious sleep. The
Echidna lives on land in the sand and is never far from danger. When danger does
arise, the echidna attempts to burrow in the sand. Siegel hypothesizes that any
twitching of the quills of the echidna in sleep would attract attention and not
be very adaptive, and so this aspect of REM is missing from its sleep behavior.
This speculation that REM changes with the predatory/prey conditions of an
environment has some grounding. In Allison's 1976 study, larger animals sleep
less and predatory animals have more REM. Also, herd and prey animals have less
Dolphins - Do they dream or not?
Early studies on dolphins (Flanigan, 1974) and other cetaceans seemed to
indicate that they didn't have REM. However, new studies (Mukhametov, 1995) now
indicate that they do indeed exhibit REM. For instance, dolphins allow half
their brain to sleep while the other may be awake. This is often what is
happening in slow-circle swimming.
OK, so all mammals dream, what about reptiles?
The common ancestor of birds and animals are reptiles, so we might expect
that instead of developing REM independently of one another, that there was a
common reptilian ancestor with REM. But if they do have REM, it must be quite
different from mammalian REM. Reptiles don't have the brain development of
mammals and don't show the extreme EEG differences that mammals do between wake
and sleep. Further, reptiles don't seem to have atonia during sleep. This means
that the three measurements, brain activity, input/output gating and
neuromodulation are all going to be quite different, if they exist at all.
Reptiles don't even have a neocortex!
Some REM characteristics have been found in reptiles, including chameleons,
desert iguanas and caimans. But the experiments all had problems that have left
the question open. Were they in REM or just awake? Does slower heart rate count
as atonia? More studies are needed to determine if REM is active in reptiles.
What we do now know is that animals and birds all exhibit something like REM
sleep. Yet some animals, such as birds and other animals, seem to only have a
common ancestor in reptiles. Is it possible that birds and mammals developed REM
independently of one another? Siegel feels this is unlikely, or less likely than
the theory of a common ancestor.
Table 1 : Evolutionary Lines
Thecodonts - Birds
- True Mammals - Nontherian Mammals - Egg laying Mammals (Monotremes)
Mammal_like_reptiles - True Mammals - Therian Mammals -
Mammal_like_reptiles - True Mammals - Therian Mammals -
References and Citations
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