Princes William and Harry say they learned this eco-friendly, cost-saving habit from their dad

While it may be somewhat of an annoyance, both to themselves and those around them, Prince William and Prince Harry just can’t help that they’ve picked up this habit from their dad — and turns out, it’s not a bad habit to have.

In “Prince, Son and Heir: Charles at 70,” a new BBC One documentary, Prince Charles’ sons admit they have adopted their father’s habit of turning out lights in rooms they aren’t in, even if it means most of the house is dark while they’re awake.

They told the BBC it’s a lesson they just can’t shake.

“He’s a stickler for turning lights off,” Prince Harry said of his father. “And that’s now something that I’m obsessed with as well.”

Prince William chimed in saying, “I know, I’ve got serious OCD on light switches now, which is terrible.”

Even Meghan Markle can’t help but notice her husband’s need to make sure no light goes unused.

“My wife certainly goes, ‘Well, why turn the lights off? You know, it’s dark.’ I go, ‘We only need one light, we don’t need like six,'” Prince Harry admitted.

But, despite the brothers seeming somewhat bothered by the task of turning lights off, they do think it’s an important lesson they learned from their father — and it’s something anyone can do to help the environment.

“And all of a sudden it becomes a habit and those small habit changes he’s making, every single person can do,” he said. “And I think it’s one of the key lessons that he taught us.”

According to Save On Energy, turning off lights you aren’t using really does help reduce emissions. A study by researchers at Imperial College London in 2010 found that turning off lights might save more energy than we think. “[A]ny reduction we make in our electricity use — for example, if everyone switched off lights that they weren’t using, or turned off electric heating earlier in the year — could have a bigger impact on the amount of carbon dioxide emitted by power stations than previously thought,” study author Adam Hawkes said of the findings.

So, Prince Charles and his sons are definitely onto something!

The father and heir has become known for his environmental policies. He’s created a number of initiatives that work to sustain the environment including his Accounting for Sustainability Project, which works to inspire more sustainable business models, and the Campaign for Wool, which works to aid sheep farmers market wool as a natural fire-retardant and sustainable fabric.

His love for the environment runs deep, as does his love for his family. In fact, the prince worries about grandchildren growing up in a polluted world.

“I am about to have another grandchild actually. I suspect quite a few of you may too have grandchildren or will do soon,” he said during a royal visit to Ghana recently. “It does seem to me insanity if we are going to bequeath this completely polluted, damaged and destroyed world to them. All grandchildren deserve a better future.”

And with Prince William and Prince Harry following in his footsteps, he’s one step closer to ensuring his passion for saving the environment will be passed on to the next generation.