In the UK, 61 organisations made a commitment to a 20 per cent decrease in working hours for all employees during a six-month period beginning in June 2022.
Additionally, the clear majority of businesses kept their full-time productivity goals.
With 71 per cent of employees self-reporting lower levels of "burnout" and 39 per cent stating they were less stressed, data from the largest four-day working week trial in the world show dramatically lower rates of stress and illness in the workforce.
There was a 65 per cent reduction in sick days, and a 57 per cent fall in the number of staff leaving participating companies, compared to the same period the previous year. Company revenue barely changed during the trial period - even increasing marginally by 1.4 per cent on average.
In a report of the findings presented to UK lawmakers, some 92 per cent of companies that took part in the UK pilot programme (56 out of 61) say they intend to continue with the four-day working week, with 18 companies confirming the change as permanent.
Research for the UK trials was conducted by a team of social scientists from the University of Cambridge, working with academics from Boston College in the US and the think tank Autonomy.
The trial was organised by 4 Day Week Global in conjunction with the UK's four Day Week Campaign.
Companies from across the UK took part, with around 2,900 employees dropping a day of work.
Organisations involved in the trial ranged from online retailers and financial service providers to animation studios and a local fish-and-chip shop.
Other industries represented include consultancy, housing, IT, skincare, recruitment, hospitality, marketing, and healthcare.
Researchers surveyed employees throughout the trial to gauge the effects of having an extra day of free time. Self-reported levels of anxiety and fatigue decreased across workforces, while mental and physical health improved.
Many survey respondents said they found it easier to balance work with both family and social commitments: 60 per cent of employees found an increased ability to combine paid work with care responsibilities, and 62 per cent reported it easier to combine work with social life.
"Before the trial, many questioned whether we would see an increase in productivity to offset the reduction in working time - but this is exactly what we found," said sociologist Prof Brendan Burchell, who led the University of Cambridge side of the research.
"Many employees were very keen to find efficiency gains themselves. Long meetings with too many people were cut short or ditched completely."
"Workers were much less inclined to kill time, and actively sought out technologies that improved their productivity," he said.
David Frayne, a research associate at the University of Cambridge, said, "We feel really encouraged by the results, which showed the many ways companies were turning the four-day week from a dream into realistic policy, with multiple benefits."
Joe Ryle, Director of the 4 Day Week Campaign, calls the results a "major breakthrough moment" for the idea of shorter working weeks. "Across a wide variety of different sectors of the economy, these incredible results show that the four-day week actually works."
In addition to the survey work, designed in collaboration with colleagues including Prof Juliet Schor from Boston College, the Cambridge team conducted a large number of extensive interviews with employees and company CEOs before, during and after the six-month trial.
Other pilots run by 4 Day Week Global in the US and Ireland - with research conducted by many of the same academics - have already reported their findings. However, the UK trial is not only the largest to date but also the first to include in-depth interview research.
"The method of this pilot allowed researchers to go beyond surveys and look in detail at how the companies were making it work on ground," said Frayne, from Cambridge's Sociology department.
In terms of motivations, several senior managers told researchers they saw the four-day week as a rational response to the pandemic - and believed it would give them an edge when it came to attracting talent in the post-Covid job market.
Some saw it as an appealing alternative to unlimited home working, which they felt risked company culture. Others had seen staff suffer through health problems and bereavement during the pandemic, and felt an increased "moral responsibility" towards employees.
"I hated the pandemic, but it's made us see each other much more in the round, and it's made us all realise the importance of having a healthy head, and that family matters," said the CEO of a non-profit organisation that took part in the trial.
However, many said shorter hours were being discussed long before Covid as a response to demanding or emotionally draining work.
The CEO of a video game studio pointed to high-profile examples of "crunch and burnout" in their industry as a reason for involvement in the trial.
Perhaps surprisingly, however, no organisation interviewed was taking part in the trials simply because technology had reduced their need for human labour.
Some companies stopped work completely for a three-day weekend, while others staggered a reduced workforce over a week.
One restaurant calculated their 32-hour week over an entire year to have long opening times in the summer, but much shorter in winter.
A few companies in the trial attached strings to the reduced hours, including fewer holiday days, an agreement that staff could be called in at short notice, or a "conditional" four-day week: one that only continued while performance targets were met.
Interviews documented how companies reduced working hours without compromising on targets. Common methods included shorter meetings with clearer agendas; introduction of interruption-free 'focus periods'; reforming email etiquette to reduce long chains and inbox churn; new analyses of production processes; end-of-day task lists for effective handovers or next-day head starts.
When employees were asked how they used the additional time off, by far the most popular response was "life admin": tasks such as shopping and household chores. Many explained how this allowed them a proper break for leisure activities on Saturday and Sunday.
"It was common for employees to describe a significant reduction in stress," said researcher and Cambridge PhD candidate Niamh Bridson Hubbard. "Many described being able to switch off or breathe more easily at home. One person told us how their 'Sunday dread' had disappeared."
For some parents of young children, a midweek day off meant savings on childcare expenses. For those with older children, it meant some welcome 'me time'.
All reported doing more of the activities they already enjoy - from sports to cooking, music making to volunteering. Some developed new interests, while others used the time to get professional qualifications.
"When you realise that day has allowed you to be relaxed and rested, and ready to absolutely go for it on those other four days, you start to realise that to go back to working on a Friday would feel really wrong - stupid actually," said the CEO of a consultancy organisation involved in the trial.
When it came to working culture, employees were generally positive, feeling more valued by their employer and describing a shared sense of purpose arising from efforts to make the four-day week a success.
However, several staff at one large company had concerns about intensifying workloads, while some at creative companies expressed disquiet over reduced worktime conviviality due to 'focus time', and argued that unstructured chat often generates new ideas.
By the end of the six-month trial, many of the managers said they could not imagine returning to a five-day week.
"Almost everyone we interviewed described being overwhelmed with questions from other organisations in their industry that are interested in following suit," said Burchell.
"When we ask employers, a lot of them are convinced the four-day week is going to happen. It has been uplifting for me personally, just talking to so many upbeat people over the last six months. A four-day week means a better working life and family life for so many people."