Appropriately enough, it was during a lunchtime run that I listened to the latest episode of Jocelyn K. Glei’s excellent podcast. It featured Alex Pang, writer and futurist, on the benefits of rest for the creative process.

He talked about a number of things, but it confirmed my belief that you can only really do four hours of focused, creative work per day. Of course, you can add status-update meetings and emails to that, but the core of anyone’s work should be this sustained, disciplined period of attention.

Four really concentrated hours are sufficient to do one’s most critical work, they’re sufficient to do really good work, and for whatever reason they seem to be the physical limit that most of us have.
In addition, he introduced terms such as 'deliberate rest' and 'cognitive momentum' which I'll definitely be using in future. A highly recommended listen.

Source: Hurry Slowly