I think it would help if you provided a bit more information. Such as, do you have any idea what the dog might be barking at? If this only happens overnight, what changes in the home that might cause the dog to bark at that time? Is it possible the animal has some sort of fear in the dark? Maybe a night lite or lamp left on overnight may help. Do you crate the dog at night? Does or did the dog have these barking fits on any other occasions you can recall? What breed is your dog? There are medications that may help or (cause) your dog to stay asleep during the overnight hours. As all situations are different, it's hard to help you from afar without more information.