Dogs have neurological activity when they sleep. This can manifest as small, jerky movements of the feet/legs, quiet vocalization, and mild tremors. If the shaking is not excessive or constant the entire duration of sleep I would not be concerned. You can videotape your puppy during an event and show your veterinarian if you are concerned at your next puppy vaccination visit.
Maybe you puppy is too cold so it shake at night
If he or she is not shaking excessively I would not worry. Some dogs just do this. They even have what I call doggymares. If the shaking is a lot call a vet as soon as possible. Hope this helped.
Is she a chihuahua? IF she is, it is normal. I'm NOT a veterinarian, but I DO have 2 chihuahuas, and my vet told me when I was worried about my first one shaking all the time, that it is the breed, they shake a lot. If yours isn't a chihuahua, I would recommend calling your vet, or searching online your pets breed, to see if it is normal. Hope I helped. Love & Light, Gina
It more than likely is he is dreaming just as humans do. If his daily activities show no cause for alarm then that's probably all it is.