To begin with, it is perfectly normal for a dog to sleep with its tongue out. The major reason behind this is dogs try to regulate their body temperature while sleeping by sticking their tongues out. This may be due to the reason that your dog is extremely dehydrated. Otherwise, dogs feel relaxed and comfortable while sleeping. You will also notice that they won't sleep for long with their tongues hanging out, and as soon as they wake up from their deep slumber or change their position, they will bite their tongue back into their mouth.