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But there are reasons to believe this was no fluke. A similar experiment at a New Zealand company in 2018 produced similar results. And research by Stanford University economist John Pencavel on British munitions plant workers during the First World War has found that past a certain point, working more decreases hourly output.

It’s not hard to think of some reasons this would be the case. Long hours cause fatigue, both physical and mental. That fatigue affects not just the last few hours of a workday, but all the hours of the next day. An employee who drags back to work after only a few hours’ rest isn’t going to be very productive in the morning. Then the next long day tires them out even more, and the punishing cycle begins again. Eventually the worker starts making little errors, slowing down and failing to take initiative to fix problems and exploit new opportunities.

Another reason long hours lead to reduced productivity is that a worker can simply run out of things to do. Some tasks can be done at any time, but some have to wait on the schedules of others. A restaurant’s delivery truck can’t just decide to deliver more food in a day in the absence of orders. A salesperson has to wait on customers’ schedules. A lab researcher has to wait for experiments to run, and so on. Workers can probably always find something useful to do with extra hours, but that usefulness diminishes with each additional hour. Eventually, they end up using Twitter or chatting with other unoccupied workers, waiting for the end of the day.