1. Respiratory tract obstruction: When the dog sleeps, the muscles of the respiratory tract such as throat and nose will relax. If the dog's respiratory tract is already very narrow, it is prone to obstruction, resulting in difficulty breathing.
2. Respiratory diseases: Dogs may suffer from respiratory diseases, such as asthma and pneumonia, which can cause breathing difficulties, especially during sleep.
3. Obesity: If the dog is too obese, the weight of the body will press the respiratory tract, resulting in difficulty breathing.