Breaking Your Work Day Into Units Or Intervals
Schwartz says that "the demand in our lives increasingly exceeds our capacity" but that "we can influence the way we manage our energy." We can do this by recognizing that humans follow an ultradian rhythm.
At the end of ninety minutes, "we reach the limits of our capacity to work at the highest level." Then we need to renew. At his blog, Schwartz recently referred to the work pattern he suggests--90 minutes of work, followed by a break--as "mental intervals." He also compared this work pattern to interval training, an excellent analogy, I think.
Over and over again I'm coming upon variations of the Unit System. I don't think the number of minutes in your unit or interval (90, 45, or even 20) is that important, so long as you focus your attention on work for a preplanned period of time and then take a break. It helps us to take advantage of the scientific knowledge of what we're physically and mentally capable of doing and provides an external support for willpower.
The Impact Of Lack Of Sleep
Schwartz also writes about what fatigue does to work effort and quality, saying "...even very small amounts of sleep deprivation take a significant toll on our cognitive capacity." This was intriguing for me, because I've just started reading a productivity guide for writers that described being too tired to write as merely an excuse. In the event that people truly are dealing with fatigue, they might be better advised to find a way to deal with that renewal issue.