Under normal circumstances, the main reason why your dog is shaking in its sleep is that they’re dreaming.
Several medical conditions may cause a dog to shake or twitch. When they’re sick, dogs can feel discomfort and react badly to the illness. In cases like these, they will shake even when they’re awake.
If your dog’s shaking looks more like trembling and shivering, they may feel cold. Usually, they will tuck their tail and curl themselves up into a furry ball when they’re chilly.
It’s not common for dogs to have seizures, but it’s definitely something you need to look out for. Seizures last longer than regular shaking and are typically characterized by stiff and rigid muscles.