Then theres the issue of cognitive capacity. Deep work is exhausting because it pushes you toward the limit of your abilities. Performance psychologists have extensively studied how much such efforts can be sustained by an individual in a given day.* In their seminal paper on deliberate practice, Anders Ericsson and his collaborators survey these studies. They note that for someone new to such practice (citing, in particular, a child in the early stages of developing an expert-level skill), an hour a day is a reasonable limit. For those familiar with the rigors of such activities, the limit expands to something like four hours, but rarely more.

Deep Work by Cal Newport