Focused attention, like a strained muscle, gets fatigued. Ericsson finds world-class competitors — whether weight lifters, pianists, or a dog sled team — tend to limit arduous practice to about four hours a day. Rest and restoring physical and mental energy get built into their training regimen. They seek to push themselves and their bodies to the max, but not so much that their focus gets diminished in the practice session. Optimal practice maintains optimal concentration.
— Focus: The Hidden Driver of Excellence (Daniel Goleman), Loc. 2186–9