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Looking into this question systematically in our laboratory, we failed to find any evidence of paradoxical sleep in the tortoise and concluded that probably reptiles in general were capable only of light sleep. Among birds, however, we start to see a beginning of paradoxical sleep, albeit very brief. In our subjects- pigeons, chicks and other fowl- this state of sleep lasts no longer than 15 seconds at a time and makes up only 0.5 percent of the total sleeping time, contrasted with the higher mammals 20 to 30 per cent. In the mammalian order all the animals that have been studied, from the mouse to the chimpanzee, spend a substantial portion of their sleeping time in paradoxical sleep. We find a fairly strong indication that the hunting species (man, the cat, the dog) enjoy more deep sleep than the hunted (rabbits, ruminants). In our tests the former average 20 percent of total sleep time in paradoxical sleep, whereas the latter average only 5 to 10 percent. Further studies are needed, however, to determine if what we found in our caged animals is also true of their sleep in their natural environments.

The evolutionary evidence shows, then, that the early vertebrates slept only lightly and deep sleep came as a rather late development in animal evolution. Curiously, however, it turns out that the opposite is true in the development of a young individual; in this case ontogeny does not follow phylogeny. In the mammals (cat or man) light sleep does not occur until the nervous system has acquired a certain amount of maturity. A newborn kitten in its first days of life spends half of its time in the waking state and half in paradoxical sleep, going directly from one state into the other, whereas in the adult cat there is almost invariably a transitional period of light sleep. By the end of the first month the kitten's time is divided equally among wakefulness, light sleep and paradoxical sleep (that is, a third in each); thereafter both wakefulness and light sleep increase until adulthood stabilizes the proportions of the three states at 35, 50 and 15 per cent respectively.

Considering these facts of evolution and development, we are confronted with the question: What function does paradoxical sleep serve after all? As Kleitman reported in his article "Patterns of Dreaming," Dement found that when he repeatedly interrupted people's dreams by waking them, this had the effect of making them dream more during their subsequent sleep periods. These results indicated that dreaming fulfills some genuine need. What that need may be remains a mystery. Dement's subjects showed no detectable disturbances of any importance - emotional or physiological - as a result of their deprivation of dreaming.

We have found much the same thing to be true of the deprivation of paradoxical sleep in cats. For such a test we place a cat on a small pedestal in a pool of water with the pedestal barely topping the water surface. Each time the cat drops off into paradoxical sleep the relaxation of its neck muscles causes its head to droop into the water and this wakes the animal up. Cats that have been deprived of paradoxical sleep in this way for several weeks show no profound disturbances, aside from a modest speeding up of the heart rate. They do, however, have a characteristic pattern of aftereffects with respect to paradoxical sleep. For several days following their removal from the pedestal they spend much more than the usual amount of time (up to 60 percent) in paradoxical sleep, as if to catch up. After this rebound they gradually recover the normal rhythm (15 percent in deep sleep), and only then does the heart slow to the normal rate. The recovery period depends on the length of the deprivation period: a cat that has gone without paradoxical sleep for 20 days takes about 10 days to return to normal.

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