Also a nice summary of how the brain learns (pay special note to the graphs):http://www.slideshare.net/ParkerLe/how-the-brainlearns
It's data from decades ago, and more geared towards learning, but probably applicable to intense focus as well.
- We learn best in 40 minute chunks. Too short and we feel rushed, too long and we get tired.
- There's a 5-15 minutes peak of focus in those periods of time (maybe needing a small ramp up time)
- a 'slow' period around 20 minutes in when you're still capable of maintaining focus, but won't really remember anything. Probably best for things like compiling.
- Another smaller 5 min peak of focus (like a second wind) near the end of the time block.
- After that, you should take a short break, from 5-30 minutes.
(maybe I should move this stuff into a tutorial on practical psychology or something