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.

Conclusions

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.

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Table 1 : Evolutionary Lines

Thecodonts - Dinosaurs
Thecodonts - Birds

Mammal_like_reptiles - True Mammals - Nontherian Mammals - Egg laying Mammals (Monotremes)
Mammal_like_reptiles - True Mammals - Therian Mammals - Marsupial Mammals
Mammal_like_reptiles - True Mammals - Therian Mammals - Placental Mammals

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): . http://www.bbsonline.org/Preprints/OldArchive/sleep.html (2000)

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.
http://www.npi.ucla.edu/sleepresearch/rem_evolution.htm

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.