Our livingroom clock has exactly that. Funnily, they run on separate batteries, and the pendulum batt gives up long, long before the clock.

There is no general answer to that. Some people can’t focus on a job from 12 to midday, others can stay hyperfocused in the zone until they drop or someone gets them out of it. As a student, I did a highly concentrated programming job from Thursday to Saturday in one go with only short breaks for food and bathroom, and three hours of sleep in total.