If you notice your young pup shivering in his sleep, the most likely cause is that the little guy is simply dreaming, but there are several other reasons for this behavior. Puppies easily can become chilled, so he just may be cold and need a warmer spot to snooze. If his shivering is constant, even while he's awake, or if he's sleeping more than usual, he could be ill and need veterinary care.
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They're dreaming
Like people, puppies dream and may shiver, twitch or even vocalize a bit in their sleep. When your puppy sleeps, he goes through periods of slow wave sleep and rapid eye movement; dreaming occurs most frequently during REM sleep. Once he settles down for a snooze, your puppy will enter REM sleep after about 20 minutes, according to Popular Science. Small dogs, like puppies, experience short periods of REM sleep, around one minute in length, which occur every 10 minutes while asleep, according to Health24. It's during these times that you'll probably notice your little pup shivering or performing other involuntary movements in response to an exciting dream.
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They're cold
Young pups can quickly become chilled if kept in a room with a draft or in extremely cool temperatures. Puppies actually develop the shiver reflex to help keep their bodies warm at 2 1/2 weeks old, but will need an external source of heat to stay cozy until they reach around 4 to 5 weeks old, recommends the Hartz website. During their third and fourth weeks of life, your little one needs an ambient temperature of around 75 degrees Fahrenheit. Provide your pup with a heating pad, set to low, wrapped in a towel so that he has a warm spot to sleep and won't shiver because he's cold.
They're sick
If your puppy is sleeping a lot more than usual and shivering, it could be because he's in pain or feels nauseous, according to WebMD Pets. He also may experience lack of appetite, vomiting or diarrhea, which can quickly dehydrate a small pup. Get your puppy to the vet right away if you notice these symptoms. In puppies, distemper is a common cause of shivering and lethargy, especially because the vaccine for this disease is first given around 8 weeks of age. This serious illness can strike pups not yet vaccinated and causes other symptoms including fever, coughing and runny eyes and nose. Your vet can treat this illness with intravenous fluids, antibiotics and dilators for his airways. Another reason, relatively rare though it can't be ruled out entirely, is that he may be having a seizure. Keep a close eye on the severity of his shaking. If the movements are small and jerky, he is likely dreaming. If his shaking is severe, his body appears stiff and he does not respond when you attempt to wake him, these are signs of a seizure and you should contact your veterinarian immediately.
Considerations
Puppies generally need 15 to 20 hours a day of sleep in a comfy, quiet spot in your home to maintain their health, according to the Eagle Valley Humane Society. If you see your little one shivering during a dream, gently pet him or speak softly to him to reassure him without waking him up. You also can give him a blanket to snuggle with if you think he's cold. If your pup is constantly shivering, even while awake, your pup may be experiencing discomfort, stress or a reaction to a medication, warns Vetstreet. Get your pup to the vet for an exam to determine the cause of his shivering.
References
WebMD: Dog Shivering and Trembling: Causes and Treatments
Eagle Valley Humane Society: Does Your Dog or Puppy Need a Nap?
Vetstreet: Why Does My Dog...Shake and Tremble?
Hartz: Let's Get Warm
Psychology Today: Do Dogs Dream?
Health24: Dogs Dream of Past Experiences
Popular Science: FYI: Do Animals Dream?
2ndchance.info: Ain't Misbehavin' -- Training Your New Puppy
PetPlace.com: Do Dogs Dream?
Sarah's Dogs: Why Do Dogs Shake When They Sleep?