The six-month pilot enlisted over 60 companies and just under 3,000 to feedback on the “100:80:100” working model: 100% pay for 80% of the time, in exchange for 100% productivity—and it has been hailed a major breakthrough.

The findings—which will be presented to the government, as campaigners urge lawmakers to give every British worker a 32-hour working week—include a 65% reduction in the number of sick days, maintained or improved productivity at most businesses, and a 57% decline in the likelihood that an employee would quit, dramatically improving job retention.

The results even found that reducing employees’ working hours had a positive impact on the bottom line.

Company revenue increased slightly by 1.4% on average over the trial period, and by a much higher 35% when compared to the same six-month period in 2021.

It’s no surprise that 92% of the companies that took part in the scheme, ranging from local fish-and-chips shops to large corporations, have decided to continue with the four-day week. 

Joe Ryle, director of the 4-Day Week Campaign, described the results as a “major breakthrough moment.”

“Across a wide variety of different sectors of the economy, these incredible results show that the four-day week with no loss of pay really works,” he said. “Surely the time has now come to begin rolling it out across the country.”

But an employment lawyer has warned Fortune that “there are many practicalities for a business to consider”; meanwhile CEOs cautioned that fewer days with the same expectations could lead to anxiety among workers, with one noting that “companies shouldn’t implement a drastic work change blindly.”

Don’t jump on the trend lightly

Although for the most part employees reported an increase in their well-being and work-life balance, for a small minority of employees this was not the case. 

“Just like any change, it will suit some and alienate others, and the reality may be that the structure doesn’t suit every employee or business model,” Pierre Lindmark, founder and CEO at management consultancy Winningtemp, says. “The truth is that the four-day working week isn’t for everyone.”

He warns that “one less day at work could lead to increased anxiety and isolation as the result of having the same amount of work to do, but less time to get it done.”

Associate Solicitor at esp Law Charlotte Morris echoes that the six-month trial simply isn’t long enough to measure the long-term implications of a shorter working week.

“Businesses may be able to sustain it for 12 months but not for five or 10 years, and, on the flip side, the positive impact that improved employee well-being can have will be best seen when you can compare staff absence, sickness rates, and burnout over years,” she warns. 

“Therefore, results must be taken with a degree of caution and they may not be sufficient enough for us to see a seismic shift to a four-day week by businesses.”

Ben Thompson, CEO of HR services Employment Hero, is also taking the “impressive” results with a grain of salt, because although the employers in the trial were cross-sector, “they all had to opt into the trial—meaning they were somewhat invested in the scheme working.”

“Some businesses, particularly those that interact with other businesses in real time, could find real difficulty in just taking one day off,” he adds.

Considerations when implementing a four-day week

“It may seem obvious, but companies shouldn’t implement a drastic work change blindly just because a trend is gaining traction,” Lindmark adds.

Instead, businesses should take a considered approach to launching a four-day workweek, with employees at the heart of their decision-making. 

“Organizations must consult their workforce before drastic changes to working practices, as it’s likely that even employees in favor of the transition will have key questions to be answered,” he advises.

There are also a host of practicalities that businesses need to consider when altering their working model. 

“Businesses can’t simply change a person’s contractual terms unilaterally,” lawyer Morris warns. 

Before making any permanent changes, she advises businesses to trial a shorter week and says “the execution of the trial must be well-advised.”

“It must be clear that it is a trial that may not be implemented permanently, and contractual agreements or proof of employee agreement to this may be required so you can revert back to a five-day week should you wish to,” she adds.

Plus there will be an abundance of contractual changes that must be made with employee buy-in, such as what happens with part-time workers who already work a short week, which day workers will be “off,” and how holiday pay entitlement is calculated.