There’s a correlation between the size of a dog and his sleep, says psychologist and dog researcher Stanley Coren, PhD. The bigger the dog, the longer the REM cycles — those portions of sleep in which most dreaming occurs.
Lest you think that means big dogs dream more than little ones, Dr. Coren also says that smaller dogs dream with greater frequency than larger dogs, even though their REM cycles and their dreams might be of shorter duration. A large dog may dream every hour or so while a small one might dream as often as every 10 minutes.
One sign that your dog is in his REM cycle is rapid eye movements under his closed lids, perhaps accompanied by muffled grunting or barking and running motions in his legs. The dreaming portion of sleep allows dogs to work through the events of their day, just as it does for people, adding greater import to the old adage, “Let sleeping dogs lie.”