WITH the prospect of Easter and two bank holidays on the horizon, Britons have an embarrassment of long weekends to savour. For Caroline Lucas, the joint leader of the Green Party, that is not enough: she wants three-day weekends every week. On March 31st she announced at her party’s conference that the Greens were exploring such a policy, which could go in their manifesto in 2020.

The Greens reckon that lopping a day off the working week would begin to redress the inequalities between men and women, since both paid and unpaid work might then be shared more evenly. People could do more of what they love (assuming that excludes their jobs) and would be less stressed. It would reduce Britain’s carbon footprint. And it could even boost the country’s productivity.

Some evidence supports this. A paper published in 2014 by John Pencavel of Stanford University, looking at first-world-war munitions workers, shows that reducing working hours can be good for productivity. Between 2007 and 2011 the American state of Utah adjusted the working week for state employees, with longer days from Monday to Thursday, and Friday off. In ten months the shift saved the state $1.8m in energy costs.

But the Greens would go further than simply redistributing working hours over four days: they suggest that people could work fewer hours overall. Since not everyone can afford to take a 20% pay cut, “wages must go up correspondingly”—courtesy of employers and the state—“to ensure no one loses out”. The 26m employed Britons earn on average £90 ($112) a day. Covering the shortfall would cost around £120bn a year, equivalent to the budget of the National Health Service.

The Greens’ proposals encounter two problems. First, the theory. They argue that the reduced hours worked by some could be redistributed to others in order to lower underemployment. They thus fall prey to the “lump of labour fallacy”, the notion that there is a fixed amount of work to be done which can be shared out in different ways to create fewer or more jobs. In fact, if people worked fewer hours, demand would drop, and so fewer working hours would be on offer.

Second, the cost. Increased productivity could cover some of the costs of paying a five-day wage for a four-day week, suggests Sarah Lyall of the New Economics Foundation, a think-tank. She points to a Glasgow marketing company that did just that, and experienced a 30% leap in productivity. But that is an astonishing increase to expect across the board.

The Greens say they are in the early stages of exploring the idea and have not yet produced firm costings. It might be useful to do so before next month’s local elections. That leaves little time—but presumably the party’s policymakers will raise their productivity accordingly.