Deliberate practice—whether for a mental or physical activity—is highly mentally demanding. Because the focus of the practice is on getting the details of the execution correct, the mental demands of practice are almost always more strenuous than the physical demands.
There are many reasons that deliberate practice is mentally demanding. Deliberate practice requires sustained, lazer-like focus in order to concentrate on specific aspects of the skill performance. The sheer number of repetitions can be exhausting, and keeping oneself mentally engaged throughout these repetitions imposes a heavy cognitive load. Finally, the activities that are performed during deliberate practice are just outside of the practitioner’s current skill set, which means that even more mental effort is required in order to develop the capacity to consistently execute the skill. In short, every element of deliberate practice imposes a mental burden.
On the positive side, the ability to endure the mental demands of deliberate practice improves as one continues to practice. Research into the practice habits of music students demonstrates that the capacity for focused practice grows as the student improves. In other words, practice results in the ability to practice more.
However, there is an upper limit to the ability of the mind to sustain the kind of effort demanded by deliberate practice. Again, the research on musicians shows that professionals max out at the ability to engage in about an hour and a half of deliberate practice. After that, the brain needs time to recuperate. Some music professionals built their schedules around two deliberate practice sessions: a morning session and an afternoon session, with a substantial break in between. These professionals could carry out at most three hours of deliberate practice a day, but often they needed to do two shorter sessions (for a total of about two hours) rather than two full-length sessions a day.
What this research tells us is that it’s worthwhile to divide our practice activities. During deliberate practice sessions, new skills can be worked on with intense focus (and with the concomitant intense mental effort). If additional practice is performed that day, it probably won’t be deliberate practice, and so the focus should be on activities that do not require that kind of intense effort (such as having warm-up dances or doing “fun” activities).