Hi, Pat. Research has shown that the optimal working period is 90 minutes followed by a 20 minute break. When you think about it, that's pretty close to a school timetable, which (in my day at least) was two 40 minute periods (with a 5 minute break for changing classrooms) followed by a half-hour break. During the work periods the class had to be in the classroom. During the breaks they had to be out of the classroom.
Using something on the same lines, adapted to your own particular circumstances, can make a good structure for your day too.
My intention isn't to be contradictory. Instead, it's only to add more information to think about. I worked in an extremely high stress occupation. Our schedule was to work a 3units work followed by 1 unit break. The formula was usually one hour work followed by 20 break. The industry did exhaustive testing which indicated that a person couldn't perform extreme stress and highly exacting thinking for more than an hour in a row. More than that and the mind would start to waver and the workers couldn't continue with efficient and exacting performance results. Of course, when we HAD to go beyond that, they'd give us a double break (40 minutes) to de-compress the accumilated stress in our our brains and our body and return to a "relaxed state" to go right back into the "pressure chamber"! LOL! In conclusion, I would recommend that you also factor in the stress levels and the degree of exacting thinking and awareness required of you when calculating an effective work/rest ratio. My peers used to "envy" my generous work/rest schedule...but even with that, our turn over rate was astronimically high as people would drop like flies in a very short period of time. To not only last longer, you want to ensure that while you are working, you are protecting your effeciency level as well.
Also, be willing to be flexible with the duration of your allotted work time. On particularly tough days, after 3 hours work, we'd be sent home yet paid for the full 8 hours shift. The CEOs wanted us to be in prime shape at the beginning of each new day. Guage your body and your mind. You don't want to burn yourself out from one day to such a degree that you pollute your effectiveness for the next day as well. Monitor yourself and be willing to be flexible to meet the demands of your work and also proper care for yourself to ensure that you can do it again tomorrow, and the days and years that follow!
Using something on the same lines, adapted to your own particular circumstances, can make a good structure for your day too.