Enabling staff to rest every two weeks on Fridays "to do sports, surf or read a book all day" increased employee performance, according to the company boss.
Cardiff's branding agency Cre8ion said the move improved its creativity and retention of employees.
The idea of a four-day week – or lowering the average weekly hours to 32 without a pay cut – is backed by labor.
A business body, the FBI Wales, said there was insufficient evidence to support the move.
For six months, the nine-person Cre8ion team, which has offices in Cardiff and Bristol, worked on a two-week scheme where employees take the first Friday in full, then work on research and development and ideas on the second, either in the office or wherever. to want.
How does it work?
"Talk to the most creative agencies, Friday afternoon is a bit of a write-off so you really really miss the morning," CEO Darrell Irvine said.
As long as employees plan ahead and know their deadlines, they "don't miss a beat", he said.
"Having that extra space helped them creatively. There are other industries that are similar, where they always think.
"It's a passion … to allow people to experiment, to let their passion flourish."
For example, staff with young children may not always do the things they really enjoy over the weekend, he said.
"Having that extra day back, letting them go and play sports, go surfing or read a book all day, which gives them that time back.
"And people may think 'well, it might not work in my industry,' so I challenge people: 'Well, if you can't lay off all your workforce on Friday, why not have a full-time workforce from Friday and the other half took Monday? & # 39;
"If you want to keep the millennia, these kinds of things are really important to them."
He said the move could improve the bottom line for companies, but a "leap of faith" is needed.
Is it real?
Proponents of the idea believe it will improve productivity – unions backed the plan, but a labor-ordered report concluded it would not be "realistic, or even desirable" to impose.
The FBI Wales said there was insufficient evidence to support workers moving into a four-day week at the same salary.
"We'd all like to spend the day surfing," said director Ian Price, but there is "no guarantee" that reducing hours to increase productivity.
He said it would hurt already strained companies and noted that many firms already offer flexibility through reduced hours, compressed hours and work from home.
What is Productivity?
Productivity is the amount of output you get from each worker and is a key factor in raising your standard of living.
Factors that contribute to productivity are complex and questionable, and are related to machinery and technology investments, for example, and employee performance.
Wales has the lowest productivity of 12 nations and regions in the UK, according to the Office for National Statistics.
The Organization for Economic Co-operation and Development says the average worker in the UK makes more hours per year than employees in Germany, the Netherlands and Denmark, but less than those in Italy, Spain and Portugal.
Germany, the Netherlands, Denmark and Spain have higher productivity than the UK, while Portugal and Italy are lower.
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