Jonathan Bartley told BBC One's Andrew Marr Show that people felt "short-changed" by the economy, which he said had brought growing inequality.
Caroline Lucas said there was "a lot of evidence" that when people are exhausted their productivity goes down.
The proposal was announced at the Greens' conference in Liverpool this week in a pitch for the youth vote.
The party has said it could be included in its 2020 manifesto.
Mr Bartley said: "We really want to flag this up because we think we need bold new ideas for the country.
"We're facing in the 21st century a very uncertain world with big pressures from corporate globalisation.
"When I was a kid we were told there would be all this wealth created, we'd have this great technological advance, but all we're seeing is growing inequality and we feel that people are being short-changed."
Ms Lucas said people were "working ever-more hours" and suffering "ever-more ill health".
She said: "What we want to do is take a step back and think what is the purpose of the economy, what kind of country do we want to be, and do we really want a future where all of us are just trying to work even harder?"
A press officer for the party said the proposal was being "looked into" and policy details would not be outlined yet.
She said there was plenty of evidence to suggest working less had positive impacts, pointing to a report by the New Economics Foundation, a left-of-centre think tank.
Among other points, the think tank has argued a shorter working week would lower unemployment by redistributing paid and unpaid work more evenly across the population.
The co-leaders of the Green Party have explained their party's proposals to introduce a three-day weekend.