The four-day workweek is slowly gaining momentum across Europe—these are the elements needed for it to succeed

There’s increasing traction for a four-day workweek in Europe. Early results from national pilots across countries like Iceland and the U.K. suggest workers feel well rested, less stressed and find a better work-life balance if they can work one day less.

But it’s clear that this change requires a real administrative commitment, is far easier for some industries than others, and could be encouraged by a robust national policy.

The successful pilots have followed the '100-80-100’ rule, where workers remain 100% productive, 80% of the time, for 100% of the pay. Employees effectively carry out the same amount of work but in less time, and crucially, many organizations haven’t seen a negative impact as a result on productivity and revenue.

Workers feel well rested, less stressed and find a better work-life balance if they can work one day less.

Workers feel well rested, less stressed and find a better work-life balance if they can work one day less.

The results aren’t as successful when employees try to squeeze 40 hours of work into just four days. The conclusion of independent studies, such as a Gallup 2022 survey, suggests this approach leads to increased burnout.

Asda, one of the UK’s largest supermarkets, discontinued a trial for staff to work 44 hours over four, rather than five days, for the same pay, because staff found the 11-hour shifts too demanding, particularly if they relied on public transport or had caregiving responsibilities.

It depends on the sector too. Industrial companies might be able to gear up with increased technology to give workers shorter shifts, but in the service sector it can be more complicated. The same is true in industries where there are huge staff shortages. It’s also more likely that jobs that can use AI to help make staff more productive will also be more likely to implement shorter working weeks for the same pay.

LONDON, ENGLAND - APRIL 29: A woman loads her food shopping from Asda in Sainsbury's bags into her car in the car park at an Asda supermarket on April 29, 2018 in London, England. Major supermarket chains Sainsbury's and Asda have been reported to be in talks over a £10 billion merger deal which would mean the possible new supermarket group would have 31.4% market share, compared with Tesco's 25.6%. (Photo by Chris J Ratcliffe/Getty Images)

Some organizations, such as South Cambridgeshire Council in the U.K., have kept the shorter workweek after trials because they believe it makes them more attractive to potential employees—they say they can’t compete on salaries alone.

National policy can play a large role too in the transition to a four-day workweek. In Iceland’s case, 86% of the country’s workforce is now working fewer hours or has the option to do so, because unions renegotiated working hours after trials finished in 2021.

Who decides which day a worker takes off, and what happens if everyone wants Friday?

In France it’s been suggested that a shorter working week could help pass unpalatable retirement reform to ease national debt. Might the French be encouraged to work to an older age if they had a shorter working week?

In Belgium, the first EU country to legislate its citizens the right to decide if they want to work a four or five-day week with the same salary, a survey found that the idea is most popular with 20 to 30-year-olds (47.8%) and might become even more popular moving forward if companies can navigate the complicated administrative issues.

In essence, it’s a big burden, legally and administratively. Who decides which day a worker takes off, and what happens if everyone wants Friday? What if a bank holiday falls on a day off and how is the process different for part-time, temporary, or senior executives?

Workers in the 1800s who worked 100-hour weeks might never have imagined that most employees would now be working just 35 to 40, so change does happen, but slowly. What’s more likely is a gradual shift to other flexible working options first—a new trial in the U.K. starts in November, looking at a shorter working week but also arrangements like a nine-day fortnight or starting and leaving work earlier or later.

This story was originally featured on Fortune.com