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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. 

Wartime munitions workers

Wartime munitionettes typically put in more than 50 hours of work per week, and sometimes as many as 72 (Credit: Alamy)

In 1915, the government established the Health of Munition Workers Committee (HMWC) to monitor working conditions and advise on matters such as working hours. The committee managed to collect a rich set of data that can tell us a lot about what happens when people work long hours.

The information is especially useful because of the nature of the work and the workers: there was only one shift, and each worker did the same tasks, which were easy to count. The committee could verify which days and hours they worked by looking at the factory's electricity logs. Because the information is unambiguous and measurable, it's great data for researchers.

Output per hour peaked at about 40 hours of work per week and then fell

The 2015 analysis of this data showed that as hours worked increased, output also increased, but only to a point. Output per hour peaked at about 40 hours of work per week and then fell.

Study author Dr John H Pencavel, a professor in the economics department at Stanford University, suggests there's a sweet spot in the number of hours people work per week. "After a point (a point that probably varies across workers and across their tasks), one more hour of work delivers more output (or better performance) if the worker has already worked 30 hours a week than if the worker has already worked 40 hours a week," he says via email.

Pencavel discusses the same munitions workers in his book Diminishing Returns at Work: The Consequences of Long Working Hours. There he explains that the workers typically put in more than 50 hours of work per week, and sometimes as many as 72. Pencavel's number-crunching shows that the weeks when output was highest were not the same weeks when the hours were longest.

This means that, at a certain point, throwing more hours at the problem doesn't help – and only runs up the operational costs.

Taking time off

It’s not just about working hours: days off are also important when it comes to productivity. Munitions workers often worked many days in a row without rest. Saturday work was still common then, and Sundays were reinstituted as workdays because of the war.

Occasionally, however, munitionettes got to take a Sunday off. The HMWC collected data covering both these conditions and realised that a work week without a day of rest doesn’t benefit anyone. Output does not increase, and workers are unhappy.

The HMWC realised that a work week without a day of rest doesn’t benefit anyone. Output does not increase, and workers are unhappy

Pencavel also compared the productivity of munitions workers during the week before a working Sunday and during the week after. When the workers worked continuously without a day off, weekly output dropped between 13.5% and 17%.  

The circumstances of the munitionettes are unique. They did manual, repetitive labour in dangerous conditions. Women outnumbered the few men and youths employed in the factories by four to one. Given that they took the jobs as a call to action to serve their country and they were paid per task, rather than an hourly rate, it's fair to assume their motivations were similar too.

Manual labourers can track their hours more easily – making it easier to measure their output. For those in offices, it can be much harder (Credit: Getty Images)

One-hundred years on, the results of overwork don't seem to be all that different for knowledge workers. Working too many hours backfires for both employers and employees, whether you measure by decreased outputs, lack of creativity, a drop in quality or poorer interpersonal skills.

There isn't a body of research pointing to long hours being the optimal condition, whereas there's ample evidence in support of shorter workweeks for all kinds of workers

That's not to say that a short sprint of long hours to finish a project or meet a deadline isn't worthwhile. And some may argue that, despite regular long hours, CEOs and other dedicated executives still manage to thrive in their roles. Although there certainly could be outliers, there isn't a body of research pointing to long hours being the optimal condition, whereas there's ample evidence in support of shorter workweeks for all kinds of workers.

People need weekends and other types of time off to recover from work and return to a job refreshed and fully productive. A study of Israeli workers who all took the same two weeks away from the office found that everyone experienced some form of relief during their time off. It didn't matter if one employee felt acute stress before the time off and another didn't. "It is the day-in-day-out, seemingly endless and inescapable nature of chronic stress that causes burnout," the authors write.

The act of going to work and working wears people down. Time off enables them to recover.

Some types of workers can't avoid long hours in a condensed schedule, usually due to the conditions of their workplace. Emergency response workers, sailors, miners, long-distance lorry drivers, surgeons and airline staff all come to mind. Studies of such groups usually focus on the effects of sleep deprivation, because working long continuous hours means not sleeping. The risks and dangers vary by profession, but are consistently negative when long working hours lead to insufficient sleep.  

For people who work desk jobs, it can be almost impossible to track your output – especially if it involves answering emails after hours (Credit: Alamy)

Setting boundaries

One advantage manual labourers like the munitions workers have over most knowledge workers is that they can track their hours worked and tasks completed more easily. It's much harder to quantify output if your job is to foster long-term relationships with other businesses, for example. For decades, time studies have shown that workers in the 20th and early 21st Century overestimated how many hours they work and underestimated their leisure time. Yet as the nature of work changes, it's even harder to figure out what constitutes ‘work’ at all.

"What can be tricky for knowledge workers is keeping a handle on how much they work," says Dr Erin Reid, associate professor in human resources and management at McMaster University in Canada. "Many of these individuals are expected to respond to work-related emails or text messages at all times of the day or weekend and often on vacation. As a result, even if they are technically 'off' work, they are still thinking about and engaging with work, which can be very draining." She says that in addition to becoming less productive, people may also lose the joy or fulfilment that they used to get from working.

Long work hours and demands outside of working hours are imposed upon employees by management and are usually a part of a workplace or professional culture – Erin Reid

Solutions for preventing overwork sound deceptively simple and involve setting boundaries and managing workloads. "But to be honest, long work hours and demands outside of working hours are imposed upon employees by management and are usually a part of a workplace or professional culture," Reid says. "Asking workers to solve the problem for themselves, on their own, can only lead to temporary and incomplete fixes." In other words, it’s up to today's employers and managers to curb overwork.

Pencavel notes that some employers know the consequences of overwork on their bottom lines and their employees, though others are not fully informed.

"They have operated a certain way for a certain period of time and, if they are still in business, they see no reason to change things," he says. Still, some employers have experimented with different work schedules, and altered scheduling practises after reviewing the results.

“Why don't we observe more employers doing this, i.e., experimenting with different hours?” Pencavel asks. “I don't know why they don't.”

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