Energized Work practice also known as 40-hour Work Week or Sustainable Pace is a practice of working only as many hours as you can stay productive. The basis for it is that software development is a creative work. Every day software developers have to build a new product - something never done previously. Burning people out making them work more hours can increase productivity in a very short time. However, in mid-to-long term tired people can hardly invent the elegant and effective solution.
Energized work keeps the developers' mind alert and pen to new ideas. Fresh mind can come up with a way better and easier maintainable solutions, than somebody working 12 hours a day.
This page is a part of the Extreme Programming overview