From Pet Health Learning Center
You know your dog better than anyone else so when they don't seem themselves then you will want to know why. Sleeping more after a long day or in the heat is normal, but if your dog suddenly begins to act differently and is sleeping more than usual then they be suffering from lethargy. If this is the case, then you should be concerned.
Most medical conditions will cause your dog to be lethargic, and this can come on both suddenly or slowly over a period of time. Either way as soon as you notice your dog acting lethargic you must take him or her to the vet as they will be able to advise why your dog is lethargic.
What causes lethargy?
There are several different diseases which cause dogs to become lethargic; in very young dogs and puppies it can be very serious. They could have one of many infections including intestinal parasites, Dog Parvovirus and Canine Distemper Virus. Your dog could also have an infection, malnutrition, low blood sugar, Dog Diabetes and/or suffering from Dog Dehydration. All of which will cause your dog to be more lethargic. They also may be displaying other symptoms such as Dog Diarrhea, Dog Vomiting and nasal discharge.
Older dogs and puppies
Older dogs often begin to slow down and will sleep a lot more than before; in some cases they are simply getting old. They are less active, sleep a great deal and loose interest in their favorite games. These signs will often simply mean they are getting older and there is nothing to worry about. Loss of hearing, sight, senility and Dog Arthritis can all affect how much your dog will want to sleep as he gets older. Hypothermia will affect older dogs and puppies because they find it difficult to regulate their own body temperature which can also cause them to be lethargic. If you are in any doubt about the cause of the extra sleeping then take your pet to see the vet.
What to do
There are things that you can do at home to try to make sure that your dog doesn't contract a disease or illness. Ensure that all water and feed bowls are washed regularly and that you give your dog plenty of fresh clean water and high quality food. You should ensure that your dog is fit and has regular exercise and is taken to the vets for routine checkups. They will be able to tell if something is wrong on one of these visits. Try to make sure that your dog doesn't roam from home, and that all of their vaccinations are up to date. With simple health care for your dog you can try to ensure they don't get ill, but if they do then make sure you take them to the vets as soon as possible.
Taking the dog to the vets
If you have to take the dog to the vet then they will do a number of tests to find out the cause of your lethargic dog. They will often do blood tests and X rays to try and establish a cause and they will often ask you a lot of detailed questions to get a full background on the dog. With all of this information and test results they will be able to treat your dog and have him back up and running about in no time at all.