Tired team members make more mistakes. Airline pilots, truck drivers and other occupations that require sharp concentration mandate the maximum number of hours that can be worked. That doesn’t apply to the software industry, and the overall quality of software reflects that fact.
Agile advocates only working overtime if it is effective, and certainly not for an extended period of time. The basis for this is to allow Agile teams to work at a pace that can be sustained for the long term without causing burnout. This improves the quality of the work performed, and improves the team’s morale.
Of the 4 variables of project management – Resources, Scope, Quality, and Time – Sustainable Pace is intended to improve the control of Quality.