Try not to work more than 49 hours a week
Records from British munitions factories during World War I showed output rising proportionally up to 49 weekly hours. Beyond that, each additional hour produced less. Seventy hours produced almost the same output as 56; the extra 14 hours added virtually nothing.
What it takes
Potentially less overtime pay, and effort negotiating with a manager.
What you may gain
Historical factory records showed roughly proportional output below 49 hours/week, slower gains above that, and maximum output near 63 hours. Output at 70 hours was nearly the same as at 56, making the additional 14 hours effectively unproductive.
Context & considerations
These century-old data concern mainly female manual workers. The threshold need not be the same for mental work; the author notes that other occupations may have higher or lower thresholds. The reliable message is diminishing returns beyond some point, not a universal cutoff of exactly 49 hours.
Research & references
Pencavel (2015). The Productivity of Working Hours. The Economic Journal. https://doi.org/10.1111/ecoj.12166