Philadelphia ? All work and no play doesn’t just make Jack a dull boy. It also may make Jack a hotter boy – and harm the planet, to boot.

Two researchers – presumably keeping their hours to a minimum – looked at how long Americans work compared with people in other nations. Then they figured in productivity in terms of gross domestic product per working hour.

Not counting vacations, Americans worked 1,817 hours (34.9 a week), topped only by Greeks and Turks. Norwegians put in the fewest hours (1,336, or 25.7 per week).

Next, researcher David Rosnick and economist Mark Weisbrot, both at the Center for Economic and Policy Research in Washington, factored in energy consumption. They found that the more hours a country’s citizens work, the more energy it uses.

Their point isn’t that work consumes more energy than leisure. It’s that more work leads to more pay (for some, anyway).

Which leads to shopping. And the more stuff we buy, the more energy it takes to manufacture, transport and use it. Think TVs, DVDs, SUVs …

“Your environmental footprint is proportional very directly to your consumption,” says Weisbrot.

The researchers determined that if workers in the 15 nations of “old Europe” shouldered U.S. hours, as some economists urge, they would use 18 percent to 41 percent more energy.

That would make it impossible for the continent to reach its carbon emission reduction goals, and warm the planet slightly.

Conversely, if Americans, who historically have gotten more pay instead of more time off, ratcheted back to European levels – opting, perhaps, for siestas and “la bonne vie” – U.S. energy consumption would decrease by 14 percent to 26 percent.

As Weisbrot jokingly put it: “Workers of the world, relax.”