Why Is My Dog Trembling While Sleeping?

Small dog sleeping with teddy bear
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Though sleep rests and restores the body, studies of brainwave patterns reveal that a dreaming mind is at least as active as an awake, alert mind. Your dog can't tell you what he dreams about but research suggests that sleeping animals weave dreams from recent waking experiences. Dogs love to run but obviously, doing so while asleep would be dangerous. To keep them safe, their brains block their muscles from actively participating in dreams. So if your dog is dreaming about chasing squirrels, the twitching and trembling of his legs and paws may represent the visible evidence.

Dogs, People and Dreaming

The brainwave patterns of sleeping dogs and people are much the same, says Stanley Coren, professor of psychology, animal behaviorist and author of many books about the canine mind. Dogs pass through the same series of sleep cycles as humans, with vivid dreams occurring during the REM -- or rapid eye movement -- stage. To some extent, how frequently REM cycles occur depends upon the size of your dog. Small dogs may have dreams every 10 minutes but large dogs have fewer dreams that last longer. An average-sized dog will start dreaming about 20 minutes after falling asleep, Coren says. That's when you'll start to see his legs twitching and his eyes darting around behind closed lids, as though he's watching something only he can see -- which he is.

The Brain Stem Inhibits Movement

All mammals dream but luckily for dogs and others, when they enter REM sleep, a section of the brain stem called the pons kicks in to partially paralyze their muscles, thereby preventing them from physically acting out dream experiences. Otherwise, a dog dreaming that he's enjoying a good romp in the park might be up racing around and barking instead of just making twitchy leg movements and funny little whimpers.

Re-experiencing the Day's Highlights

If an ingenious experiment conducted at the Massachusetts Institute of Technology has general application, your dreaming dog may be reliving -- and thereby creating permanent memories of -- all the fun he had that day with you. Researchers at MIT's Center for Learning and Memory compared the individual waking and dreaming brainwave patterns of rats trained to find their way through a maze to win food treats. After examining more than 40 REM episodes in the sleeping rats, researchers discovered that about half the brainwave patterns were almost identical to those recorded while the same rats were in the maze. Authors of the study, published in the January 2001 edition of the journal "Neuron," concluded that while asleep, the rats relived that event to create and preserve a permanent memory of it.

Rehearsing for Threatening Situations

Since dogs and people have similar dream cycles, and people sometimes have bad dreams, it seems reasonable to assume that dogs do as well. However, if your dog's dreaming sometimes appears a little distressed, that might be healthy too. According to "threat simulation" theory, throughout evolution, one key function of dreaming may have been to simulate virtual realities of dangerous situations that dreamers might encounter in waking life. This allows the dreaming mind to rehearse tackling them without physical risk, thereby increasing preparedness. So if your dog considers protecting his home and humans to be his sacred duty, troubled dreams might be his brain's way of rehearsing strategies in which his heroic actions save the day.