There are several possibilities that could cause your dog to tremble in his sleep. More serious causes could be that the trembling is a symptom of pain, injury or disease.
Just like people, dogs dream. A dreaming dog may move his legs in a running motion. The dog may also twitch, vocalize and even tremble. Both humans and dogs have a stage of sleep known as rapid-eye-movement sleep, or REM sleep. It is so named because during this stage of sleep, the eyes can be seen moving beneath closed lids. This is the stage of sleep during which the most vivid dreams are known to occur.
Dogs often tremble when they are in pain. If your dog is elderly, arthritic or has recently been injured, and the trembling is a recent phenomenon, pain should be a consideration. You can discuss the possibility with your veterinarian and see if a pain medication might be suitable for making your pet more comfortable. A heating pad beneath a towel can often provide some comfort to an arthritic pet, as well as a bed with plenty of stuffing to give old bones a soft place to rest.
There are some illnesses that may cause a dog to tremble. In most instances, the trembling associated with illness would occur when the dog is sleeping or awake. Distemper is one of the most serious illnesses associated with trembling. Symptoms of distemper include runny eyes and nose as well as fever. Depending on the severity of the trembling, epilepsy could be a cause. Some disc, nerve and neurological issues can also result in trembling.
The trembling may be nothing more than a sign that your dog is cold. This is especially a possibility if your dog is a young puppy, away from mom and litter mates for the first time. Provide your dog with a cushioned bed, away from drafts or cold air. You might even try covering the dog with a blanket at bedtime. If he is cold, he is likely to leave the blanket in place. Another option would be to offer your dog a place snuggled up close to you in your own bed.