“Deep practice, however, doesn't obey the same math. 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.”