Here’s a theory prevalent in the start-up community and among CEOs: passion, focus, ambition and dedication are crucial to being your most productive self. With these traits, we can always push ourselves to do more ­– or so the thinking goes.

Research on working hours, however, suggests that overwork leads to being less productive, not more. It is also associated with increases in cardiovascular disease, diabetes and other negative health effects, all of which can take a toll on work-related output.

So how many working hours is too many? And by how much does productivity drop after a certain point? The only way to know is to measure the output of real workers. A 2015 analysis of unusual worker data from World War One can offer some concrete answers.

Sweet spot?

During the war, Britain needed to manufacture weapons and ammunition quickly. Because so many men were off fighting, women – who had not typically worked in factories at the time – were called into service as "munitionettes". Their work included tasks like drilling screw threads on parts of weapons before they were assembled, and turning fuse bodies, which likely meant turning a piece of metal as a machine cut it and bore holes in it.