My cat does that whenever she's in a deep sleep. I'm talking the kind of sleep where her whole body goes limp. She starts twitching and you can left her head or paw and drop and she doesn't wake up. I thought something was wrong with her the first time I encountered it and finally ask my vet about it.
She said that my cat felt safe and secure enough to allow herself to fall into a deep sleep and that the twitching was nothing to worry about. She only does it when she is in a deep sleep. Well it's been 12 years and she's still alive and somewhat healthy so the twitching at least in my cat's case is harmless. I'm not sure if this applies to dogs as well. |