The study, carried out by the University of Sussex and learning tech firm Thrive, looked at MRI scans, blood tests, and sleep tracking to go with the productivity gains. 

The results, released Thursday, were overwhelmingly positive, as expected. General well-being was up 21%, including less stress and emotional exhaustion. Employees felt more accomplished and confident over the duration of the study.  

“Our findings suggest that multiple aspects of mental and physical health can improve for staff if they work a 4-day week. In the long run, these will add up to better health over the lifespan – especially if staff remain on the 4-day week,” Charlotte Rae, an associate professor at the University of Sussex and the research lead of the four-day workweek project, told Fortune. 

It all makes for a sound case to adopt a shorter week, right? Except that’s not what Thrive is doing. Despite the benefits, the company said it won’t implement a four-day week on a full-time basis because there was a need for a “widespread cultural change across the U.K.” 

“As a business serving hundreds of organisations, it highlighted that five-day coverage for our customers is essential when they’re operating more traditional ways of working,” Thrive’s co-CEO Cassie Gasson said. The firm’s 115 employees participated in the trial. 

In other words, no matter how good a four-day workweek proves to be, it’ll never be good enough unless businesses and governments collectively advocate for and adopt it.

Four-day workweeks have dominated the workplace conversation as countries and companies look for new ways to boost productivity, motivation, and employee engagement. The U.K. has conducted those experiments across different groups and varying lengths of time with resounding success. 

Most four-day workweek set-ups involve a reduction in hours, but not in pay. Yet, some companies like German multinational Bosch are using it as a cost-saving mechanism. 

For those who have tried it, the advantages are clear and undeniable. They’ve yielded success, whether they take place in Iceland or Japan. But then comes the important hurdle: is it feasible, and is there a strong enough business case for it?  

The four-day workweek has gone from being a one-off trial to an actual policy consideration recently in a new flexible working bill. Under the proposal, employers must consider requests from their staff for compressed hours or flexible timings, which the government hopes will change Britain’s productivity game.   

There’s no dearth of challenges in business, whether that’s high costs, labor shortage, or tough policies. That’s prompted some companies to take a strict approach focused on the bottom line rather than giving room for flexibility. 

That’s not to say companies aren’t leaning towards a shorter week. Most of the 61 companies participating in a six-month pilot project in the U.K. decided to stick with it. The framework is, however, doesn’t apply to some industries like retail. 

It’ll also take many more trials to pivot away from the decades-old practice of working close to 40 hours a week.

“The 4-day week – or other versions of it, such as a 9-day fortnight, or shorter days – is already a reality for many employees in the UK, with several hundred employers now operating this,” Rae said. “For this to be more widespread, and for society to gain the potential economic and health benefits, we need more trials in a wider range of sectors.”