You did not mention how old your dog was, or whether your dog was male or female, or spayed or unspayed. This information, and how often this occurs, is very important in offering suggestions as to the cause of this symptom. Symptoms such as incontinence can be accompanied by other, less obvious, symptoms that could indicate a serious underlying health concern. For this reason, your dog should have a thorough examination by your veterinarian.

In young or very old dogs, the occasional accident while sleeping is common, and should not cause concern unless frequent or accompanied by other symptoms. As in humans, these age groups have less control over their bladders.

In adult females, the most likely cause of such a symptom is a Urinary Tract Infection (UTI), which is caused by a bacterial infection of the urinary tract. A simple urine test will be able to determine if your dog is suffering from a UTI and treatment is nearly always successful. A second possibility is a condition that affects only spayed female dogs. Sometimes a hormonal imbalance can lead to a weakening in the tone of the sphincter muscle, and in turn leading to occasional urine leakage. This is often successfully treated with Phenylpropanolamine, which is sold under the brand names Propalin and Proin.

Adult males can also suffer from Urinary Tract Infections and poor tone of the sphincter muscle, although these are far less common in dogs. Another possible reason is an ectopic ureter, where the ureter opens in the urethra instead of the bladder, causing a constant trickle of urine, which is particularly obvious when sleeping because the dog is stationary. If you suspect this might be the cause of your dog’s symptoms, check the area under your dog after he has been sat in one position during the day, and consult your vet if your believe this to be a possibility. A condition exclusive to adult males is prostate disease, but unlike in humans this is very unlikely to be cancerous. Swelling of the prostate in dogs will also result in pain and difficulty during defecation, and sometimes your dog will walk slightly differently. Affected dogs with advanced prostate disease will also begin to suffer from blood or pus in their urine.