Does a dog breath heavy when asleep?

There are multiple reasons for labored or difficult breathing in your dog, ranging from your dog just ran around the yard like crazy to pneumothorax. If you can't find a reason for your dog to be breathing heavily (exercise, hot and/or humid environment), you should definitely keep a close eye on him and possibly take him to a veterinarian.

Some breeds are known to have breathing issues, particularly if they are in hot environments and haven't had surgery to shorten their soft palate - these are the brachycephalic (smashed-in face) breeds like pugs, Boston terriers and Maltese. If taking these types of dogs into an air-conditioned environment doesn't improve their breathing in a few minutes, you need to get them to a veterinarian.

Other causes for heavy breathing include infection, cancer, fluid in the lungs from any of a number of causes and trauma to the rib cage or chest. Depending upon the severity, any of these might need immediate veterinary care.