The Science of productivity:

Shockingly, when we look at some of the most elite musicians in the world, we find that they aren’t necessarily practicing more but, instead, more deliberately. This is because they spend more time focused on the hardest task and focus their energy in packets — instead of diluting their energy over the entire day, they have periods of intense work, followed by breaks. Not relying on willpower, they rely on habit and discipline scheduling. Studies have found that the most elite violinists in the world generally follow a 90-minute work regime, with a 15- to 20-minute break afterwards.