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In the lesson one you say: "Productivity varies over the course of the workday, with the greatest productivity occurring in the first four to six hours. After enough hours, productivity approaches zero; eventually it becomes negative."

But later you say: "Crunch Mode can increase output over the short term".

How can these two sentences be true at the same time? How exactly long is this "short term"?

And another question: what's working time? I'm sitting here at work at the moment and waiting for a collegue releasing a software piece I need.

On the one hand its overtime because I cannot spend this time on my hobby activities or just resting. On the other hand I'm obviously not working.

Evan Robinson

Productivity varies with daily hours both in the course of a day and over days and weeks.

In the course of a day your first work hour is generally less productive than average as you get back into it, then your middle hours are most productive, then as you tire you get less productive. So your first and last hours are generally less productive than your middle hours. If you do enough hours (as an extreme example, 24 hours or 48 hours straight), you get tired and stupid enough to make mistakes. If you make enough mistakes, your total work can become negative (imagine being tired enough to accidentally wipe out the source control tree).

Over time (days, weeks, and months), your productivity can rise or fall with the number of hours you have been working. If you work at or below your daily recovery rate, your productivity will not vary much over these longer times, but if you work enough hours so that each day you are a little tireder than the day before (or worse, so that you are not recovering on the weekends so that each week you are a little tireder than the week before) your daily productivity will also decrease over days, weeks, and months. Eventually you get so tired that you make mistakes.

The way Crunch Mode can increase total output for the short term is by understanding the above and its implications. Let's make a simplifying assumption and say that your first eight hours a day are at a fixed productivity and that productivity drops 10% for each hour you work after eight hours. That means that in 9 hours you get 8.9 hours work done (8 hours at 100% = 8 hours plus one at [100% - 10%] = 90% = 0.9 hour). At 10 hours you get 9.7 hours work done (8 + .9 + .8). At 16 hours you get 12.4 hours work done (8 + .9 + .8 + .7 + .6 + .5 + .4 + .3 + .2).

So if you crunch hard for a short period of time (short enough so that the days, weeks, months productivity hit doesn't get you), you can get more than 8 hours work done in a day. You just don't get 16 hours work done because someone works 16 hours. The extra hours are less efficient. If you absolutely need three weeks worth of work done in two weeks, you can do it this way. But the longer-term effects prevent you from getting three years work done in two years by working extra hours, or even three months work done in two months. The limits will vary depending upon how many extra hours you're working. Check out the Revay Report for a collection of graphs and data on how much productivity loss you get for a given number of additional hours.

As for what constitutes work time, I'd have to fall back on common sense. As a simple approximation, time awake and at the office, but obviously some time will be more tiring than other time. Sleep deprivation is a critical problem, and if you're not getting a good 8 hours or so (human average requirement, YMMV), all your time is less efficient. Short of that, think of the differences between sitting on the couch and playing soccer. Your body gets tireder playing soccer. Your mind gets tireder more subtly: work can make it tired, but so can boredom.

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