Your dog lies asleep at your feet, and suddenly his legs begin to twitch and run. Is he dreaming? Or are you dreaming to think such a thing? Wake up and smell the dog food! Of course he's dreaming. Why would you think otherwise?

Although no one really knows the true function of dreaming it does seem to be necessary for normal data processing and memory storage. Dogs think and they have memory. And their memory banks need period purging and reorganization during sleep just as ours do.

Dogs and humans are not as different as some scientists would have us believe. Scientists tend to dwell on the differences between the two species, whereas the sameness is positively compelling. We're 95 percent identical genetically and physically (right down to the iron-containing porphyrin ring our common blood pigment, hemoglobin). Because of this blue print similarity, you might expect a lot of the inner workings to be the same –and they are. Our brains are similar, our neurochemistry is the same, and our reflexes and memory are "wired" in like manner.

Types of Sleep

Like humans, dogs have two main types of sleep: rapid eye movement (REM) sleep and slow wave sleep (SWS). As a dog falls asleep the first stage he enters is SWS, the "sleep of the mind," in which mental processes are muted but muscle tone remains. The next stage is REM sleep, the "sleep of the body," in which the body is fully relaxed but the mind is racing and the dog's eyes are darting rapidly.

In SWS, brain waves are slow, undulating and of high amplitude much like those in a lightly anesthetized animal or person. In this stage, the dog appears calm and at rest. Dogs and humans are more easily aroused from SWS sleep, which appears to be a transitional state with incomplete muscle relaxation.

By contrast, in REM sleep brain waves are rapid and irregular, like those of the awake state. Dogs, like people, display REM sleep, and during REM sleep they show evidence of heightened mental activity – fast electroencephalogram [EEG] (brain wave) pattern. They may move their legs as if they are running, may whine or whimper as if excited, and may breath rapidly or hold their breath for short periods.

When REM sleep is achieved they are at their most relaxed and are most difficult to waken. It is during this more profound physical sleep that their eyes begin to dart and the brain waves pick up pace. Humans awakened from this state report that they have been dreaming; dogs are probably dreaming too when they are in REM sleep, although no dog has ever told anyone about a dream he's had.

Incidentally, adult dogs spend about 10 to 12 percent of their sleeping time in REM sleep. Puppies spend a much greater proportion of their sleep time in REM sleep, no doubt compacting huge quantities of newly acquired data.

And if you've ever wondered whether dogs that seem to be running during sleep are dreaming of catching rabbits or something similar, you can safely say they are.