When you're feeling rundown, you may oversleep — and dogs are the same. "Many dogs sleep an average of 18 hours a day," Fratt says. "But if your dog is slower to rise than normal or is less excited about walks than normal, you might have a problem." Again, this is partially about knowing what's typical for your dog. If a dog who loves to take a nap and then run outside and play seems lethargic after a nap (and maybe even takes another nap shortly after), that could be a sign of depression.