This is Hidden Brain. I'm Shankar Vedantam.

In 2012, Michael Phelps was at the peak of his career.

At the London Games, he became the most decorated Olympian of all time.

He's coming hard, Phelps. He's still a chance. He's a real big chance. Can he do it again?

He hits it and he does. Remarkable, stunning.

He had 22 Olympic medals to his name, including 18 gold medals.

The swimmer's entire life had centered around his sport at being the best.

Early mornings, endless training.

He pursued victory tirelessly and it paid off.

The Olympic record, Michael Phelps and his team have maintained this extraordinary record.

But then, it was over.

No more early morning practices, no more races, no more gold medals to chase.

He described the feeling as a post-Olympic depression.

I saw myself as strictly a swimmer, not as a human being, he said.

At times, he felt like he did not want to be alive.

Eventually, the great athlete turned to advocacy, using his experience to raise awareness about mental health.

He realized that retirement was scary because he had to find, quote,

whatever it was, I was looking for.

Michael Phelps' experience illustrates a challenge many of us face.

When the rules we've relied on to live a good life stop working, where do we find new rules?

How do we discover what we are looking for?

Over the next couple of weeks on Hidden Brain and in a companion episode on Hidden Brain Plus,

...