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The strict on/off cycle is a useful psychological tool for shutting out distractions. The 25/5 cycle helps me keep focused, since if I'm getting frustrated on something, I know I don't have to spend more than about 20 minutes on it before I can stop- but I can't give up until then, which has me working through a lot of blocks that would usually send me off towards Google Reader, a horrific time-sink from which I often never return. For getting generally tired or that slowly-growing urge to check blogs and/or a forum or two, the 20-minute breaks seem "close enough" that I can keep myself on track until then.
I guess I need this extremely rigid system, or else I get frustrated, get distracted, and never get back to work- or I get hyperfocused on something and don't realize when I've stopped being effective due to fatigue.
20/10 sprints (work for 20 minutes, take a 10 minute break) never worked for me. I'm not sure what's different. I guess it's the rigid tracking system of Pomodoro, in part- I'm not "working for 20 minutes, then taking a break", I'm "working on something for 25 minutes, then unloading and switching tasks." On second thought, I think that's the more important thing: I'm using none of the 25 minutes to fret about what I should be working on. If I don't already have a task list, then my next 25 minutes is spent building my next set of tasks.
This has been orders of magnitude more effective than David Allen's "Getting Things Done". The free 14-page PDF beat the $10 240-page book. GTD is too heavyweight for what I really needed, which was a way to not spend more time context-switching than working. Dedicating a 25-minute run every so often to fully update my lists is a lot more effective than an elaborate tracking system just to maintain the Next Actions queue.
Comparing it to 20/10 sprints- I think the other difference is break schedule. 5/5/5/20 is actually equivalent to 10/10/10/10 for total time spent not working, but I find 5/5/5/20 works quite a lot better for me. Five minutes is long enough to pee and cool off my brain, but not long enough for me to get interested in something else and then reluctant to actually get back to work at the end of the break- and for those things, 20 minutes is long enough to let me finish it and feel satisfied with the time off. I can check Twitter in the five minutes, but not much else. (This post, for example. Whoops, 1min 23sec left, wrapping it up now...)
I hope this keeps working for me. It's certainly going a lot better than pretty much everything else I've tried for keeping focus.