In general, most dogs sleep in one of two ways: Either curled up on their side or stretched out on their back. If you have ever seen your dog sleeping on the floor, it’s likely that he did not choose this position. He may have been forced by an outside force, such as being kicked over while playing or having a pillow yanked from his mouth when he was asleep. When a dog goes to sleep, he stops breathing. This is because his respiratory system is at rest. When he sleeps, his body temperature drops. Because of this, his blood vessels constrict, which prevents him from sweating. Sweat glands open only during physical activity, not when the dog falls asleep. This is where sleeping with the tongue out comes into play. Since dogs aren’t able to sweat, they must find another way to keep themselves cool.