Panting at night is generally normal for a puppy, but consistent heavy panting may indicate a health problem or even something psychological. Because panting is a normal behavior during any time of day or night, though, don't leap to conclusions just because you hear him breathing loudly in the next room. Investigating a few probable causes of his panting, you may prevent a trip to the vet.
Dogs of all ages pant as a way of cooling down. While humans eliminate excess body heat through the surface of their skin by sweating, dogs and puppies are covered in fur that prevents such relief. They sweat through the uncovered pads on the bottoms of their paws, and they hang their mouths open and pant. Their exposed tongues allow them to effectively expel body heat.
Just like humans, dogs breathe may breathe heavily when under stress. As the heart rate elevates and body temperature rises, a dog's respiration rate can increase, leading to panting. If your puppy pants at night, he may be suffering from nighttime anxiety, which could strike for a number of reasons. For example, if you don't share your bedroom with the puppy, he may feel separation anxiety that makes him pant when he's left alone all night. He may be spooked by headlights outside, as well. Monitor your puppy's nighttime behavior to determine if something in particular gives him anxiety and causes him to pant. Consult a vet and consider socialization and basic training work.
The temperature and humidity inside your home generally fluctuates between day and night, and this may cause your puppy to pant more during some periods than others. For example, if you turn up the heat or turn off the air conditioner at night and the temperature in your home rises, your puppy's body may struggle to adapt and leave him panting. Meanwhile, running a humidifier at night can make it more difficult for him to breathe. Take special note of any such atmospheric changes in your home between day and night.
Potential Health Problems
Unfortunately, ceaseless panting may indicate health problems, especially if your puppy can't stop panting long enough even to sleep at night. For example, Cushing's syndrome is an adrenal condition that can also cause hair loss and extreme thirst and hunger. Your puppy may suffer from a respiratory disorder like pneumonia, particularly if he wasn't kept warm enough after being born -- younger puppies need warm conditions. Monitor your puppy's panting behavior day and night for several days and, if he doesn't seem to stop, take him to a vet as soon as possible.