Concentration allows mediocre hours to produce exceptional depth.

Concentration compounds within a session. Two focused hours can go deeper than a whole day of interrupted work, because you reach a state where the material is fully loaded and connections start appearing. An unremarkable amount of time can produce remarkable depth if it is continuous.

Rather than trying to find more hours, try protecting the ones you already have from interruption. Measure output by depth rather than duration. The gain from continuity is usually larger than the gain from more time.