Spending more time is effective—but only if you're still in the sweet spot at the edge of your capabilities, attentively building and honing circuits. What's more, there seems to be a universal limit for how much deep practice human beings can do in a day. Ericsson's research shows that most world-class experts—including pianists, chess players, novelists, and athletes—practice between three and five hours a day, no matter what skill they pursue.
