Bernie Sanders just threw his support behind the 4-day work week. Here's why that isn't a perk everyday workers actually want.

• Four-day work week trials across the world have been successful for both employers and workers.
• But that's mostly for white collar workers, a union leader said at the World Economic Forum.

More and more people are advocating for a four-day work week as the next frontier for a better work-life balance. Sen. Bernie Sanders threw his support behind it this week, citing a UK-based study.

But many of those people are likely white collar workers. Christy Hoffman, General Secretary of the UNI Global Union, explained why during a panel about the movement for a four-day work week at the World Economic Forum in Davos, Switzerland, last month.

The UNI Global Union is an international organization representing millions of employees in the skills and service sectors. Hoffman said that her experience advocating for these workers has taught her that four-day work weeks solve less of their problems than those in higher paying jobs.

"Nobody's talking about the four-day week in my world," she said. "Flexibility is what everybody wants."

Speakers on the panel, which was hosted by Adam Grant, an organizational psychologist at the University of Pennsylvania, included Sander van 't Noordende, the CEO of Randstad, an HR services provider, and Karien van Gennip, the Dutch Minister of Social Affairs and Employment. Their experiences span corporate, politics, and advocacy, but they agreed that flexibility was key to attracting — and keeping — workers in a climate where they're still prepared to quit, or at least quiet quit.

But when it comes to the four-day work week, that's "an upper-class issue," Gennip said.

Flexibility for lower-income workers, according to Hoffman, involves reliable scheduling, breaks between long shifts, and more vacation time.

"They're happy to do a 15-hour stretch, but they want an eight-hour break"

Recent studies show that four-day work trials have been big successes.

One global study involving 33 companies that switched from five-day weeks to four found that revenue increased over 8% overall in the six-month trial period. Additionally, respondents reported less burnout, less fatigue, and an increase in physical health.

In one massive UK pilot involving 70 companies over a six month period, the organizations involved increased productivity overall, and reported similar benefits to employee wellness. Workers had more time for leisure and exercise, and were more productive during their reduced hours.

"If your home life is great and you're balancing home life and hobbies, you're going to come into work happier," Fiona Blackwell, associate director at Girling Jones, a construction-recruitment firm in England, told Insider after a few months into their four-day experiment.

So among the workers whose jobs can accommodate a four-day system, such a change works out for them and their employers.

But that only represents a fraction of working environments, Hoffman said.

Service workers like grocery store employees "want to know when they're working the next week," she said. "They want to know when they can take their child to the doctor's… algorithmic management has come into their lives so they no longer work every Tuesday or Thursday."

Algorithmic management is when technology and data collection are used to make management decisions. Such sporadic scheduling is the norm at stores like Kroger's, which is the largest grocery chain in the US. In one study among 10,000 unionized Kroger's workers across the country last year, a quarter of respondents said that they were given a day or less notice about schedule changes.

"Scheduling is really important for low-wage, predominantly women workers," in service industries, Hoffman said.

In other cases, she added, people in industries like film or video game production are often hit with long hours during crunch time. Workers in film production might spend 15 hours a day on set, Hoffman said, and video game programmers might be on the clock for 24 hours at a time.

And it's not that these workers would change the long hours — it's that they want a full eight-hour break between these stretches when they pop up, Hoffman said.

"Tech workers want to be able to take the next week off" after working so many hours in a row, she explained. "We have Amazon workers who do four-day work weeks, but ten hours days, and they would prefer five eight-hour days."

It's a population of workers searching for a different sort of flexibility, she said — especially when switching to a four day week would be impossible given the constraints of their work, or would necessitate the challenge of getting pay increases to compensate for the lost days.

And it's why more vacation time seems like a better starting point for many workers in terms of getting them their leisure time, Hoffman said. That's especially true of the US, which is one of only a few countries with no guaranteed paid time off.

"Some people would rather have a five-day work week and have six weeks off," she said.