Zoning Out Clock

At what point do the “Yeah” and “Uh-hum’s” fade in as your mind fades out. Is it a certain conversation that starts the fade, the way I speak, or just the mode your mind is in? Maybe it’s the time there is something else riding in your mind or the time your mind simply wants to shut me off?  I don’t know all I know is you’re not a great actor as the zone-out period is as obvious as that daze on your face.   If I could somehow figure out where or what causes that clock to begin maybe I can stop it before I throw my clock against the wall.