The trial only ran for a month, but the results show overwhelming benefits for employee health and wellbeing.
Countries around the world have been experimenting with four-day working weeks over recent years, as academics, governments, and companies gauge the benefits and potential downsides.
Spain has been at the forefront of experimenting with four-day weeks, with a major countrywide trial currently ongoing for participating companies.
Results and analysis are in, however, for a smaller trial that was run alongside the larger one, with 360,000 workers living in Valencia taking part.
Unlike the majority of the other four-day week trials around the world, this one was just a month long - but the results show a number of the benefits that are increasingly being understood to flow from working shorter weeks.
Valencia City Council ran the trial between April and May of this year, taking advantage of the fact that there were already three holiday days within those dates - meaning only one additional day was removed to make it a month of four-day working weeks.
Following the short trial, those taking part reported a 35 per cent reduction in stress levels, with 64 per cent increasing the amount of sleep they got.
There was more socialising too, with 72 per cent saying they spent more time with friends, whilst 37 per cent said they increased their physical activity, and 46 per cent increased their time spent reading.
Valencia also saw some environmental benefits during the trial month, with a 9.5 per cent decrease of traffic in the city, and 58 per cent less NO2 concentration in the air.
The council reported that the hospitality sector and companies in the leisure industry saw the benefit of people using their extra free time to go out to eat or drink, with more people making trips within the city. Tied into this was the one downside highlighted in the report - a slight increase in the amount of tobacco and alcohol consumed.
“Our main aim with this project was to generate new evidence of the potential benefits that shorter working hours could have for urban environments,” said Joan Ribo, former Valencia mayor and speaker for the Compromís electoral coalition at the City Hall.
“The trial has been an enormous success and has provided significant empirical evidence of the positive impact of the four-day week on workers' health and city-wide dynamics. We have seen a sharp decrease in traffic intensity and air pollution, a broad improvement in overall health perception by workers and more free time dedicated to cultural and leisure activities.”
He added that the trial findings are a “new milestone” for the four-day week movement in Spain and beyond.
Four-day week trials have shown promising results in other countries as well.
The largest trial to date, which ran in the UK, included 33 companies and almost 1,000 employees. None of the participating companies planned on returning to a five-day week once the trial had resumed. They found productivity remained stable while employee satisfaction saw a dramatic increase.