catch somebody napping
catch (one) napping
To exploit or capitalize on one's inattention (or, sometimes, literal sleep). We were able to sneak into the building because we caught the security guard napping. I scored a goal after I caught their defenseman napping.
catch somebody ˈnapping(informal) find somebody not prepared or not paying attention, and perhaps gain an advantage over them as a result: Chelsea’s defence was caught napping in the final moments of the game when Jones scored his second goal for Liverpool.
Nap means ‘sleep’, usually for a short time and especially during the day.