According a recent biography of Henri Poincaré,
Poincaré … worked regularly from 10 to 12 in the morning and from 5 till 7 in the late afternoon. He found that working longer seldom achieved anything …
Poincaré made tremendous contributions to math and physics. His two-hour work sessions must have been sprints, working with an intensity that could not be sustained much longer.
I expect most of us would accomplish more if we worked harder when we worked, rested more, and cut out half-work.
Update: To be clear, the quote above refers to Poincaré’s most intellectually demanding work. Poincaré carried on his administrative duties outside the four hours of concentration mentioned above.