Think about a week, any given week where you were 'working' (I always feel the urge to use inverted commas for 'work', because for some of us work is fun).
Now that you have a week in mind, have a think about the number of creative jobs, or pieces of creative work of substantial size if job for you means something larger than a week, that you successfully got to done in that given week. These could be things like "Wrote a blog post", "Created a Screencast", "Created an iPhone/iPad logo for a client", "Created that piece of functionality" but you're probably thinking a little small if you're thinking "I wrote that tweet".
So, how many do you have? My theory du jour is that you probably don't have many than 4 discrete pieces of work, creative in nature, that require a decent level of thought and concentration, that you got to done in a given week as shown in the graph below.
(Created using Autodesk Sketchbook and Omnigraffle on the iPad.)
The theory's foundations are simple. There are roughly 4 productive full days in a week (Friday afternoon, really!?) and I suspect that there is a natural cadence for an individual to complete, at most, 4 creative tasks to match the 4 productive days. In fact, I suspect that we feel more and more unfulfilled the less we do from 4, and the more stressed out we get when things are over 4 (or worse, we are so swamped in uncreative busy-tasks that we don't feel we've done anything).
Of course, this is all just suspicion (and definitely not a license to let your boss know you've "already done your allotted 4 creative tasks for this week" on a Tuesday). The flip side of that is that if you're a manager and you have some tasks that involve this sort of deep thought, it might pay to be aware of this natural cadence and structure the workload as best you can.
In a nutshell, for creative, heavy-thought-required tasks the theory seems to follow for the past 10 years of my life, roughly speaking. Yes, there are times when I've sacrificed everything to get something like 10 discrete pieces of creative work done, but that definitely delved into having no life and stress levels bordering on the dangerous.
So have a think and see if the theory of 4 works for you. I'd love to hear your findings!
