Scientists claim perpetual energy machines are impossible.
They obviously don't have puppies.
Sensible organisms tire after exercise, but puppies do not. In fact, their energy levels go up in proportion to the amount of exercise they receive.
Ignoring this fact, Dagon and I go for long walks. Upon return, I am tired and he is, well, not. But for the first few weeks, he let me sleep. Unfortunately he has since learned to wake me up by biting my hand or nose or foot.
No sleep till Brooklyn.