[LPT] About 4 hours of concentrated work is the most optimal work load per working day

The last paragraph sums it up well:

"... we’ve come to believe that world-class performance comes after 10,000 hours of practice. But that’s wrong. It comes after 10,000 hours of deliberate practice, 12,500 hours of deliberate rest, and 30,000 hours of sleep."

Source: http://nautil.us/issue/46/balance/darwin-was-a-slacker-and-you-should-be-too