Mark Divine is a U.S. Navy SEAL and the founder of SEALFIT, a fitness organization for those who want to push themselves to the max.

Divine is no stranger to stress and extreme circumstances. And there's one practice he says has risen above others as a method to quickly and reliably help him become centered and focused.

According to Divine, "I first learned to control my breathing while practicing martial arts during my SEAL training, and the techniques I was taught proved invaluable later during the chaos of combat."

Ancient mystics and Navy SEALs agree: Your breath is the key to focus and calm. Divine recommends one particular practice to generate deep body relaxation paired with an alert mind. He calls it "box breathing."

According to neuroscience, the state of being Divine describes is extremely valuable because it combines alertness with relaxation. Or, as Divine puts it, box breathing "has a neutral energetic effect: It's not going to charge you up or put you into a sleepy relaxed state. But it will, as mentioned, make you very alert and grounded, ready for action."

Here's how you do it:

Expel all the air from your chest, and keep your lungs empty for four long counts. Inhale through your nose for four long counts. Hold the air in your lungs for four long counts. (Divine adds: "When you hold your breath, do not clamp down and create back pressure. Rather, maintain an expansive, open feeling even though you are not inhaling.") Exhale smoothly through your nose for four counts. That's one "rep." Divine recommends doing the practice for five minutes minimum to experience the benefits.

You don't have to wait for the perfect circumstance to do it, either. Divine says, "I practice it in the morning, before a workout, while standing in line, while I'm stuck in traffic, and whenever else I can." He also recommends "training" with it by doing it for 10-20 minutes at a time, and then doing "spot drills" during the day.

Whether it's a difficult conversation with your partner, a challenging workout you're about to do, or a major work presentation, there are circumstances in your life where you would be well-served by showing up as deeply grounded yet alert and focused.

"When I perform box breathing, even just for five minutes, I am left with a deeply calm body and an alert, focused state of mind," Divine says.

The action in your life may not be a black-ops extraction, but you can learn from the best to become your best.