That’s why L’Oréal, Asos and Nike have all joined the growing list of companies that are allowing staff to leave work between midday and 3.30 p.m. on Fridays during the summer months.

The concept isn’t anything new, dating as far back as New York’s advertising industry in the 1960s, when executives would leave work early to drive out to the Hamptons.

But in recent years, “Summer Fridays” have spread way beyond the Big Apple as countries including Iceland, New Zealand, Japan, and the U.K. embrace the four-day working week. 

In Britain alone, firms that reduced their working week to 32 hours experienced a 65% reduction in the number of sick days taken, improved productivity, and a 57% decline in the likelihood that an employee would quit—dramatically improving job retention.

So for employers who can’t afford to permanently reduce staff’s working hours while maintaining full pay, scaling back the working week just in the warmer months when business tends to be quieter anyway could be an attractive alternative. 

Summer Fridays: Productivity gains

The research is clear: long working hours are detrimental to our mental and physical health. 

So any wellbeing and productivity benefits gained from working a shorter week would be negated by asking staff to work four longer days to compensate for the time off.

But when businesses truly empower staff to work less it’s not just good for their wellbeing, it’s also beneficial to businesses: Much like what adopters of the four-day week trial found, businesses like Buffer that have tried “Summer Fridays” reported increased productivity. 

“From a psychological perspective, condensing our working hours can create a sense of urgency, in which procrastination might seem less attractive,” says psychotherapist Eloise Skinner, founder of The Purpose Workshop and author of But Are You Alive?.

“Shortened working hours can also have the benefit of enabling us to work when we’re most energized and engaged—thereby increasing our productivity—rather than leaving our tasks until the very end of the week, where exhaustion levels might be higher and energy is dipping.”

Experts have previously echoed to Fortune that encouraging staff to front-load their work and maximize productivity earlier on in the week can reduce burnout and Sunday night anxiety.  

“In a world that often prioritizes efficiency, production and output, summer hours can give a sense of respect, personalization and independence to the workplace, recognizing that our lives move in seasons and that different times might call for different working patterns,” Skinner adds.

But why restrict flexibility to the summer?

Although many businesses are eyeing up “Summer Fridays” as a morale-boosting alternative to a four-day working week, experts warned Fortune that the popular perk could have the opposite effect. 

With longer daylight hours in the summer, workers have plenty of time to enjoy their evening and “get their required dose of Vitamin D”—even if they work until past 5 p.m. 

“Is summer the time when you least need to cut back worker’s hours?,” Andy Brown, CEO of Engage poses to employers.

With the night arriving not long after 3 p.m. in the depth of winter, Brown believes that employees would benefit more from flexibility in the winter to make the most of the shorter days. 

“We know that during the winter months, with the reduced daylight, we are more likely to suffer low moods, with some people suffering from seasonal affective disorder,” Amrit Sandhar, CEO and founder of The Engagement Coach echoes. “Whilst it might be nice to have Fridays off during the summer months, it’s actually during the winter months we are more likely to need the support.”

Plus, being confronted by grim weather, shorter days and what feels like eternal darkness come winter, is no easy feat—without the added sting of having to adjust to working longer hours again.

Really, Sandhar believes that the difficult transition back to full-time work may even cancel out the benefits of ever having a reduced week in the first instance.

A bit like working from home, employees may be resentful if they’re stripped from the benefit of working reduced hours despite showing that they could be just as productive if the perk continued once the summer is over.

“In the run-up to winter months, it’s possible that this could cause greater disengagement than having never offered it at all, especially once we’ve allowed it to become established as a way of working, albeit for five months of the year.”

Do you want to attract talent or appease clients?

When weighing up whether summer hours would be a suitable substitute to scaling back worker schedules, it’s worth employers asking themselves whether talent attraction or client availability is their top priority.

Realistically, scaling back to a four-day workweek could reduce the available time clients have to interact with staff by 20% each week.

“We are hearing a lot of evidence from CEOs in client-facing sectors that the huge increase in flexible working means that clients are starting to get frustrated with not being able to get access to people, advice and responsiveness,” Brown warns. “At least with summer hours, clients can get some availability to employees on all five days of the working week.”

Having launched a four-and-a-half-day week at his own firm, Sandhar agrees that everyone in the company has had to adjust the way they work. For him personally, that has included turning down any meetings on a Friday morning that could result in work overflowing into the afternoon. 

Plus, as Brown cautioned, maintaining the shorter week has required bringing their suppliers and clients on board with the changes. 

But if the only major drawback of launching a four-day week over summer hours is that flexibility could be a year-around nuisance for clients to contend with, it’s worth remembering that year-around flexibility is equally a major benefit for most workers. 

As Sandhar says: “If the objective is to attract and retain the best talent, with the choice between working for a company offering four-day working hours, compared to summer hours, it would be little contest.”