Occasionally, the player's Pokémon will take a rest from Core Training, indicated by the Pokémon lying on its side with a "..." displayed above it and a refusal to participate in a Super-Training Regimen (or accept a new training bag); tapping the Pokémon will wake it up and give it a second wind (shown as a smiley above the Pokémon), which doubles the speed at which the Pokémon can finish off a training bag. - Bulbapedia
So just don't tap the screen for a while and stay in the Super training screen, and your Pokémon will eventually fall asleep.
I don't know why you'd want this if you're trying to get Reset Bags, though. Going to sleep won't make you find them any faster. Just keep tapping until one appears, and then it will be beneficial to go to sleep, since then you can finish punching it in half the time.
Hope I helped!