Don't keep adding to your weekly work hours: past about 49 hours, each extra hour adds less and less

High value
Evidence B
Saves time
Costs a little
Takes minutes
Some willpower

In plain words

Records from British munitions factories in World War I show that up to 49 hours a week, output rose with hours worked. Beyond 49 hours, each extra hour added less and less. Working 70 hours a week produced almost the same as working 56, so the extra 14 hours were wasted.

Cost: You may earn a little less overtime pay. It may also take effort to talk to your boss.
What you get: Output records of British munitions workers in World War I: up to 49 hours a week, output was proportional to hours. Beyond 49 hours, the growth in output began to slow. It peaked at about 63 hours. Output at 70 hours was almost no different from output at 56, so the extra 14 hours counted for nothing.
Notes: The data come from factories a hundred years ago, doing physical work, with mostly women workers. This threshold may not be the same for mental work, and the author himself says other kinds of work may be higher or lower. What is reliable is the direction: past a certain point, working more adds almost nothing. The specific number 49 is not reliable.
Evidence grade: B : Supported by research but without a precise number, or based on small or single studies.
Sources: Pencavel (2015). The Productivity of Working Hours. The Economic Journal. https://doi.org/10.1111/ecoj.12166

Translated from the Chinese original. Original book, section 3, tip 13. Content may be out of date and rules differ by place. When you actually deal with an agency, a doctor or a court, follow what local officials, doctors and lawyers tell you. Disclaimer