Do your dogs know the clock?
Nero does. I'm working from home a few days a week and could set the clock to his sleeping habits. They'll get a walk in the morning and then pretty much sleep all day. But at 4pm on the spot, day by day, a black lab nose nudges my right elbow to remind me that he now slept enough and wishes to be entertained. He will give me another 30 mins or so to finish whatever I was doing but then he'll come back and turn it up a notch. Are your dogs creatures of routine too? Rox isn't at all... she just goes with the flow