Dog Tired?

My dogs are constantly taking time to nap. Usually they curl up on the sofa or in their bed. But sometimes they zonk out in the funniest positions or places! It's not because they're bored. They have a zillion toys, each other, our family to interact and play with, and a large yard to run, watch squirrels and birds... 
but they instinctively know when it's time to rest, and they listen. Are we listening to our bodies? Do we know when it’s time to rest? Or are we running ourselves ragged?