The 'obsessive' habit Meghan Markle has quickly noticed in Prince Harry.

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With Prince Charles’ 70th birthday a mere two days away (November 14th), Prince Harry and Prince William have been playing the role of the doting sons.

They’ve previously shared how much they admired his strongly-held environmental values, and it’s something the boys have inherited from their father as well.

Sharing the habit he's adopted from Prince Charles, Prince Harry admitted he too is bit of "a stickler" for light switches.

"He’s a stickler for turning lights off," said the 34-year-old in the BBC documentary Prince, Son and Heir: Charles at 70.

"And that’s now something that I’m obsessed with as well."

And it's a habit older brother, Prince William has inherited too.

"I know, I’ve got serious OCD on light switches now, which is terrible," he said.

Growing up royal with Prince William and Prince Harry... they're a bit cute.

Despite this, it seems like his obsessive tick hasn't quite caught on with Meghan Markle.

"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'," he said to his brother, Prince William.

An aversion to sitting in dark rooms aside, which we can't really blame her for, Meghan did make an effort to showcase environmentally friendly brands and designers on her royal recent tour of Australia, New Zealand, Fiji and Tonga, and it's this 'little by little' approach, Prince Harry made an effort to highlight.

"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 [our dad] taught us."

But considering the size of Kensington Palace, being a stickler for energy conservation certainly would go a long way.

Are you also a "stickler" for turning off the lights? Or are you a bit guilty of leaving them on for too long? Tell us in a comment below.