I wanted to add this link because it is a great description of how anyone suffering from a chronic illness eventually starts to use as a strategy for living. It is interesting because I believe it used to be a part of general human consciousness before people started to believe that we could just run like machines, 24 hours a day, 7 days a week, with ocassional off days for maintenance. The Victorians in particular saw personal energy as limited and something to be carefully meted out. Of course, this didn't apply much or at all to lower classes, and was misapplied to middle class and upper class women, who had to reserve all their energy for child-bearing and rearing. That aside, there is a certain logic to acknowledging that no one can do it all, and for many of us there are additional challenges that sap the day's energy away.
Hope it is helpful.