The first 4-day workweek study looking at long-term effects shows that we waste a day

That’s according to new research which shows that workers can get as much done in a 33 hours week as in 38 hours. 

The report from nonprofit advocacy group 4 Day Week Global—which is the largest of its kind and the first to examine the long-term effects of the four-day week—found that the longer people worked a four-day week, the shorter their work weeks became without output or productivity taking a hit.

Workers could shave 5 hours off their workweek

Up until now, most studies examined the short-term effects of working a shorter week. 

Earlier this year, Britain completed the world’s largest trial of the four-day week, at that time. The 6-month pilot enlisted over 60 companies and just under 3,000 workers to feedback on the “100:80:100” working model: 100% pay for 80% of the time, in exchange for 100% productivity. 

The results were 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.

Similarly, the 4 Day Week Global report examined workers in the U.S., Canada, Britain and Ireland over the past 18 months and revealed that after just 6 months of working a four-day week their burnout, general health and job satisfaction improved.

Workers were given a paid day off a week but the same workload to see whether they could get as much done working more effectively—and the study confirmed they could.

Not only that but unlike previous studies of its kind, the report also highlighted that workers were able to cut their average work time by about four hours to 34 hours a week in that time.

This is because workers cut out inefficiencies that a more lengthy work week allowed, like meetings, to dedicate more time to uninterrupted focus work; Essentially, those of us on a five-day week are filling up our days with time-wasting activities. 

Meanwhile, those who continued with the schedule for a year, shaved a further hour of their work week and, as such, reported better work-life balance and a further uplift in their mental and physical health.

Plus it’s not only employees who gained from a shorter week, the four-day week was also an organizational win: Revenue increased by 15% over the course of the trial, weighted in accordance with company size. 

It’s probably why no organization expressed a desire to return to five days post-trial and 89% of workers also wanted to stay with the new four-day plan.

The perils of moving to a four-day week

In theory, shifting to a four-day week looks like a no-brainer for businesses. But in reality, experts previously warned Fortune that it’s a logistical nightmare that could make some staff members miserable. 

Associate solicitor at esp Law, Charlotte Morris, explained that “businesses can’t simply change a person’s contractual terms unilaterally.” Before making any permanent changes, 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 is calculated.

What’s more, 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, told Fortune. “The truth is that the four-day working week isn’t for everyone.”

He warned 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.”