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The five-day workweek is so engrained in business tradition that it's hard to imagine it ever changing. But what if those cherished three-day weekends that pop up a few times a year were to become standard practice? According to advocates of a four-day workweek, the results could be lower job stress, less burnout, better employee retention, and even higher productivity.

Some companies are giving it a try. "Savvy employers are catching on to the fact that employees are increasingly demanding better work-life balance and the opportunity to get work done at non-traditional places and times. The four-day workweek is a perfect example of that," Jim Link, the chief of HR for North America at global recruitment agency Randstad, told CNBC.

Wildbit, a Philadelphia software company that creates tools like Postmark, has been experimenting with a four-day workweek, with Fridays off, for the past year and a half. "When we reviewed our first year of four-day weeks, we realized we launched more features than the previous year," company co-founder Natalie Nagele told Fast Company. "The real value of a four-day week comes from healthy pressure and forced downtime. Since we know we only have four days to get our work done, we work smarter to avoid distractions and cut through the procrastination."

Of course, Americans once worked horrendously long hours, with 100-hour weeks common in industrial work in the late 19th Century. Labor unions began demanding shorter hours at the turn of the century, which started to become the norm after Henry Ford established a five-day, 40-hour workweek for his auto plants in 1926. Congress made it the law in 1940.

But since then, the American workweek has crept back up, thanks to constant digital connectedness, demanding industries, and the growth of contract work. In a 2014 Gallup poll, workers surveyed said they work an average of 47 hours a week, with many salaried workers saying they put in 60 hours. America's most notorious workaholic, Elon Musk, tweeted last month about his companies SpaceX and Tesla: "There are way easier places to work, but nobody ever changed the world on 40 hours a week," prompting a social-media backlash.

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Indeed, Musk has become a poster boy for the perils of overwork. Excessive hours are taking a toll on health and family life, according to Jeffrey Pfeffer, a professor of organizational behavior at Stanford Graduate School of Business. In his book Dying for a Paycheck, published last March, Pfeffer points to work stress as a leading cause of chronic illness, including cardiovascular disease. "I want this to be the Silent Spring of workplace health," said Pfeffer in an interview about his book for the business school's website. "We are harming both company performance and individual well-being, and this needs to be the clarion call for us to stop."

Pfeffer cites a term coined by an academic colleague, "social pollution," to describe the effects of excess job stress on life outside the workplace. "The work hours that companies are demanding of their employees are causing the breakup of marriages, burdens on raising children, and general disruption to family life. And the family unit is an important source of social support."

Can companies shorten the workweek and stay competitive? The job-search site FlexJobs performed an analysis of 50,000 U.S. companies' job postings to see which ones offered flexible work. Among those fields most likely to be open to a shorter workweek, according to the study: sales, computer and IT, medical and health, customer service, and education and training.

Experiments with a shorter workweek have become a global phenomenon. Jan Schulz-Hofen, founder of the Berlin-based project-management software company Planio, introduced a four-day week to the company’s 10-member staff earlier this year. “It is much healthier and we do a better job if we’re not working crazy hours,” he told Reuters.

“I didn’t get less work done in four days than in five," Schulz-Hofen said, "because in five days, you think you have more time, you take longer, you allow yourself to have more interruptions, you have your coffee a bit longer or chat with colleagues. I realized with four days, I have to be quick, I have to be focused if I want to have my free Friday.”

While a move to the four-day workweek is generally seen as a cutback in hours rather than pay, the ad agency Grey New York reportedly launched a program earlier this year to allow staff to work a four-day week for 85% of their full-time salary.

Companies experimenting with a four-day week say that its takes some adjustment, including a reduction in meetings and more advance planning. At Wildbit, the company realized they needed to make some modifications in their schedule, with some workers getting Monday off and others taking off Friday, for consistent coverage. Said co-founder Nagele: "We can't really tell our customers we're closed on Fridays."

Steve Koepp is a co-founder of From Day One. Previously, he was editorial director of Time Inc. Books, executive editor of Fortune and deputy managing editor of Time