Panting is a normal behavior for dogs, particularly when they are overheated or energetic. Panting helps cool them off when they are extremely hot or engaged in a very rigorous and fast-paced activity, like running. Still, if a dog--and especially a puppy--pants excessively during the night, it could be indicative of a more serious health problem.
The best way to gauge whether a dog's nighttime panting is cause for alarm is by taking note of the frequency. As this is such a normal behavior both day and night for dogs, especially younger dogs, many owners may overlook it or view it simply as an annoyance.
Consistent panting and labored breathing could be a sign of any number of different health conditions. Nighttime anxiety is common for dogs not accustomed to being separated from their owners for long stretches of time. Humidity can cause dogs to pant excessively and potentially develop heat exhaustion or heat stroke.
In the worst case scenario, heavy panting could be an indication of a condition like Cushing's syndrome, which causes intense thirst and hunger, or a respiratory disease such as pneumonia, according to The Daily Puppy. The latter can develop due to a puppy not being kept in a warm place soon after birth.
If a dog's panting is frequent and consistent, and one suspects something may be seriously wrong, one should see a vet for further information.