In the UK, dozens of companies took part in the world’s largest trial of a four-day work week.

And it was so successful that the vast majority of companies who gave it a try won’t be going back to five-day work weeks.

In fact, it was so successful that 15% of employees who participated even went so far as to say that “no amount of money” would tempt them to go back to a five day a week schedule.

Around 3,000 employees were involved in the trial, which was organized by advocacy group 4 Day Week Global, in collab with research group Autonomy and researchers at Boston College and the University of Cambridge.

The trial was flexible – companies could decide how to spread out their employees 32-hour weeks (most did so by just axing one day a week), but they had to keep their employees at 100% pay.

When the trial concluded, employees reported a variety of benefits related to sleep, stress, personal lives and mental health. Revenue stayed about the same during the trial but rose 35% on average when compared to the same period from previous years, and resignations decreased.

56 of the 61 companies who participated will be keeping the four-day work weeks after the trial ended. 18 says the change will be permanent. Two companies will be continuing the trial longer – and only three companies returned to their previous schedules.

Senator Bernie Sanders recently shared the study on Twitter and called for an end to the grueling five-day work week, citing a significant rise in productivity and employee wellness from shortening the week.