A nice quote from a letter I came across on the Hacker News from id Software co-founder John Carmack:

Using your time effectively is very important, and there is often a non-linear relationship between the amount of time you can stay focused and the amount that you can learn or accomplish. It is often possible to get more done in a highly focused 12 hour stretch than in a normal 40 hour work week that is interspersed with email, chat, and other distractions. Someone that can be completely obsessive about something does have an advantage, but the same questions about focus apply for any amount of time you choose to devote to an undertaking. Most people work at only a fraction of their potential.