Most companies involved in the world’s largest trial of a four-day working week have made the policy permanent, according to a new report.

Of the 61 British companies that took part in a six-month pilot in 2022, at least 54 (89%) said the policy was still in place, while 31 firms (51%) said they had permanently switched to a four-day working week.

The companies involved were invited to take part in a follow-up study one year on from the world’s biggest trial of a shorter working week to date.

The results, published Wednesday by think tank Autonomy, showed that all the consulted project managers and CEOs said a four-day week had a positive impact on their organization, with more than half (55%) describing the impact as “very positive.”

The vast majority (82%) of surveyed companies reported positive impacts on staff wellbeing, while 50% saw positive effects on reducing staff turnover and nearly one-third (32%) said the policy had noticeably improved recruitment.

Juliet Schor, the report’s author and a professor of sociology at Boston College, described the one-year results as “excellent,” and said the findings showed the positive effects of shorter working hours were “real and long lasting.”

“Overall results have held and in some cases have even continued to improve,” Schor said in a statement.

“Physical and mental health, and work-life balance are significantly better than at six months. Burnout and life satisfaction improvements held steady. Job satisfaction and sleep problems nudged down a bit, but the bulk of the original improvement remains.”

The report found, however, that staff in firms where the additional day off was only weakly guaranteed, or provided on the condition of meeting certain targets, had some concerns.

For example, four-day working week models that were highly conditional resulted in some staff reporting added stress to meet deadlines, resentment against others and feelings of inequity between employees based on the nature of their role.

The report added that less committed forms of the four-day week policy also left staff less able to plan activities on their day off.

The report said employees tended to use their day off for caring responsibilities, hobbies and chores, with staff keen to prioritize quality time for the weekend.

“One year on from the results of the UK’s four-day week pilot, virtually every company we’ve spoken to has decided to stick with the four-day week,” Will Stronge, director of research at Autonomy, said in a statement.

“The improvements in physical and mental health, work-life balance, and general life satisfaction, as well as the reductions in burnout found at the end of the trial have all been maintained one year on,” he added.