Where's your battery level? It might be the best question you can ask to gauge your mental health.

That's the advice from David Ko.

His new book, Recharge, challenges readers to give their mental health the same attention they give their cellphone battery.

"Everyone looks at their phone battery. Some people want to keep it charged all the time at like 90%. Some people will take it all the way down or put it in the yellow. And so we wanted to give you some tips and techniques in terms of making some of those parallels," says David.

David is the CEO of the Calm meditation and wellness app. You'd think he'd be the master of all things Zen. He's not. He's a guy with a big tech job, a family and all the stressors of normal life.

He overheard a friend ask her kid "how's your battery" rather than saying "how are you" and the power of the analogy struck him. 

David says, "People look at their phone over a 150 times a day at a minimum. They're constantly looking at it. And so every once in a while, check on yourself when you pick up that phone, and if you need to take a break, take a break."

David's book features interviews with everyone from rapper Macklemore to former Apple CEO John Sculley to to former NFL player Carl Nassib. 

You'll learn practical strategies to top off a low mental battery and develop mental resilience.