Dogs are predators - predators have to conserve as much energy as possible to survive - they conserve energy in order to hunt for food - we feed our dogs so that takes away the need to hunt but the basic genetics are still apparent. If you are well fed then you should make the most of that energy source and conserve it by resting as much as possible. Wild canids spend as much time as possible sleeping and this is true of all predatory mammals, birds, reptiles and even insects. As long as your dog is willing to get up when you pick up the lead and is happy and excited to go out it is a very good sign that your dog is content to let you protect it and make the right choices for it's wellbeing - your dog trusts you - that is a happy dog - a dog that is constantly on the go is a dog under stress.