After 20 seasons with the Los Angeles Lakers and five NBA championships, Kobe Bryant retired from basketball in 2016. But the 40-year-old still wakes up before the sun to go to the gym at 4 a.m., he told podcast host and best-selling author Lewis Howes on an episode of "The School of Greatness."

There are a few reasons the retired athlete maintains such a strict workout routine: "When I first retired, I let myself go a little a bit. ... And then the challenge was, can I get back in shape?" Without an end goal, like an NBA Championship or an MVP award, "it's really hard," added Bryant, who now runs a venture capital fund with business partner Jeff Stibel.

To motivate himself, Bryant set specific, measurable goals. "For me, it was, 'OK, I have to aim for something,'" he told Howes. "So I said, 'I want to aim for size. I want to aim for bulk. So that's a tangible thing. I'm going to go for that.'"