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 in reptiles.
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 throughout life."
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 REM.
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
||Egg laying Mammals (Monotremes)
References and Citations
- Allison, T. and Cicchitti, D. Sleep in Mammals: Ecological and Constitutional Correlates. Science 194: 732-734. 1976.
- Allison, T. and H. VanTwyver. The Evolution of Sleep. Natural History 79: 57-65, 1970
- Allison, T., H. Van Twyver and W. R. Goff. Electrophysiological studies of the echidna, Tachyglossus aculeatus. I. Waking and sleep. Arch. ital. Biol, 110:145-184, 1972.
- Flanigan, W. F. Nocturnal behavior of captive small cetaceans. I. The bottlenosed porpoise, Tursiops truncatus. Sleep Res., 3:84, 1974.
- Flanigan, W. F. Nocturnal behavior of captive small cetaceans. II. The beluga whale, Delphinapterus leucas. Sleep Res., 3:85, 1974.
- Hobson, J. A, Pace-Schott, E. and Stickgold, R. Dreaming and the Brain: Towards a Cognitive Neuroscience of Conscious States Behavioral and Brain Sciences 23 (6):
- Mukhametov, L. M. Paradoxical sleep peculiarities in aquatic mammals. Sleep Research, 24A:202, 1995.
- Mukhametov, L. A., A. Y. Supin and I. G. Polyakova. Interhemispheric asymmetry of the electroencephalographic sleep patterns in dolphins. Brain Res., 134:581-584, 1977.
- Mukhametov, L. M. Unihemispheric slow-wave sleep in the amazonian dolphin, Inia geofffrensis. Neurosci. Lett., 79:128-132, 1987.
- Siegel, J.M. The evolution of REM sleep. In Handbook of Behavioral State Control., Lydic, R and Baghdoyan (Eds.), pp 87-100, CRC Press, Boca Raton, 1999.
- Zepelin, H. Mammalian sleep. In: Principles and Practice of Sleep Medicine edited by M. H. Kryger, T. Roth, and W. C. Dement. Philadelphia: W.B. Saunders Company, 1994, pp. 69-80.
- Zepelin, H. and A. Rechtschaffen. Mammalian sleep, longevity and energy metabolism. Brain Behav. Evol., 10:425-470, 1974.