A good dog owner will usually take notice and start to worry when her dog begins to display any abnormal behaviour. However, not all strange canine behaviour is as obvious as raging aggression accompanied by seizures; it can be as subtle as sleeping longer or more frequently than usual. Although all dogs begin to slow down and rest more with age, there are some medical conditions that may cause your dog to sleep too much.
Believe it or not, many veterinarians believe that dogs can get depressed just as humans can. Canine depression can even be the result of a chemical imbalance in the brain. However, it is often caused by a sudden change in routine, such as moving to a new home, being adopted, or losing a long-time companion. In addition to increased sleeping, the primary symptoms of canine depression are decreased activity, lethargy, hair loss, decreased appetite, and weight loss. Treatment may include various behavioural or environmental treatments---such as providing your dog with a new companion or adjusting his living situation---or a medication such as Prozac.
The thyroid gland of a dog with hypothyroidism doesn't produce enough T3 and T4 hormones, causing a decrease in metabolic function. Most of the time, this condition is caused by an autoimmune response that attacks the thyroid, but it can also be caused by other conditions, such as cancer. The decrease in metabolic function causes the whole body to slow down, and, therefore, excess sleepiness, lethargy, and mental dullness are some of the most common symptoms of hypothyroidism. Other symptoms may include weight gain, anaemia, hair loss, various skin and coat disorders, slowed heart rate, and intolerance of cold temperatures. Treatment involves the life-long administration of thyroid hormone medications.
While juvenile-onset diabetes is not unheard of in dogs, diabetes primarily affects older canines, particularly females. In dogs with diabetes, either the pancreas produces too little insulin, or the body is unable to use the insulin it does produce. Symptoms include sleepiness, lethargy, increased (sometimes insatiable) thirst, frequent urination, blindness, weight loss, and an overall sickly appearance. Most of the time, treatment is the same as for humans with diabetes: insulin injections. Some breeds, such as schnauzers, various small terriers, and poodles, are at increased risk for diabetes, and, as with humans, obese dogs are also at greater risk.
There are many infectious diseases that can cause your dog to sleep much more than usual and/or act lethargic. Some of these include rabies, distemper, parvovirus, Rocky Mountain spotted fever, Lyme disease, and ehrlichiosis. It should be noted that most infectious diseases that cause lethargy and sleepiness are accompanied by a range of other symptoms that are often more recognisable.