Dogs can have nightmares. An owner may notice leg twitches, jerking, growling or barking when the animal is sleeping. Subtler and quieter movements may indicate a good dream.
A dog's brain is structurally similar to a human's, and a dog goes through the same stages of sleep as humans. A dog may have nightmares because of certain stresses or changes in his life. Nightmares, however, may occur for no apparent reason.
As a dog falls asleep, breathing becomes rapid. After about 20 minutes of deep sleep, a dog may enter the period of REM sleep. During this period, dreams occur in humans and animals with complex brains.