Riding my bike makes me happy. It always has. And somewhere along the way that turned into a question I have always wanted to answer for myself: what actually keeps a mind healthy? Before I started pedaling across Europe, I flew to Seoul to put that question to someone who has spent more than sixty years on it. Dr. Lee Si-hyung is a 93 year old psychiatrist who built his career around serotonin, the brain chemical that steadies our mood, and I got to spend several days with him: interviewing him, eating at his table, and seeing how he actually lives.
He practices every single thing he preaches. He walks every day. He eats slowly. He keeps working, writing, and seeing people at 93. He even gave up his driver's license on his own when he felt his reflexes starting to decline, nobody had to ask him. He is one of the healthiest people his age I have ever met, and I got to see it all work right in front of me.
Dr. Lee trained in psychiatry and neuropsychiatry at Yale after medical school at Kyungpook National University in Daegu. Back in Korea he became one of the country's best known psychiatrists. He has authored and co-authored more than 100 books. The best known are Live with Guts, from 1982, which stayed on the bestseller list for one hundred weeks and sold about two million copies; Serotonin-hara, his 2010 book that put serotonin into everyday Korean conversation; and Happy Aging, his latest. His work helped get hwa-byung, a Korean illness caused by years of suppressed anger, recognized internationally by psychiatry, and he has been honored with the National Decoration of Korea in 2011 for his work on forest therapy, along with academic awards from the Korean Neuropsychiatric Association. At 75 he founded Healience Seonmaeul, a wellness village in the mountains of Hongcheon built around prevention instead of treatment, and at 77 he founded the Serotonin Culture Center in Seoul.
Getting to Yale in the first place took nerve. Korea was still rebuilding after the war, and when he asked his dean for a recommendation letter, the dean turned him down because the idea seemed impossible. So he handled it himself. This is how he tells it:
"So, I took matters into my own hands. I wrote the recommendation letter in English myself, walked into the dean's office, managed to get his signature, and boarded a plane to America with nothing but sheer determination. In a way, poverty and war gave me the kind of reckless courage and opportunity I otherwise never would have had."
Dr. Lee Si-hyung
It was at Yale that he found the question that shaped the rest of his career: why does medicine wait for people to get sick before it starts caring? That question eventually became his life's work on prevention, daily habits, and serotonin.
"To be completely honest, I didn't start out with a grand, lofty dream of becoming a doctor. My family came from a long line of traditional schoolmasters, and with my older brother away serving as a student soldier, the responsibility fell on me, the second son, to support the family. Living through the poverty and chaos right after the Korean War, my priorities were very practical: I just wanted to make sure we had food on the table and find a way to be genuinely useful to others. That is what led me to medical school."
"Once I began my studies, however, I realized that the human mind is just as mysterious and complex as the physical body. Having witnessed so many people suffering from the invisible scars of the war, I felt a deep calling. I knew I wanted to become a psychiatrist, someone who could heal broken spirits, going beyond merely treating physical wounds."
"As I treated the patients coming through our doors, a thought suddenly struck me: Is there a way to prevent these people from ending up in the hospital in the first place? Why do we wait until people fall ill to spend massive amounts of time and money trying to fix them?"
I asked him to explain serotonin for someone who has never thought about it before.
"We feel an intense rush or sense of achievement when dopamine fires up. On the other hand, noradrenaline triggers anger or aggression during a crisis. Serotonin acts like a gentle brake in the middle, keeping these two opposing forces from running wild. It's not about intense, short-lived excitement; it's the hormone that helps us maintain a calm, peaceful state of mind."
Dr. Lee Si-hyung
Two interesting things I learned from our conversations changed how I think about my own days. First, the serotonin your brain makes during daylight is the raw material for melatonin, the sleep hormone it makes at night. Spend the whole day indoors away from natural light and your brain simply has less to work with when it is time to sleep. Second, about 90 percent of the body's serotonin is made in the gut, and Dr. Lee was careful to be accurate about what that means: gut serotonin does not enter the brain directly, but a healthy gut talks to the brain through the nervous system and helps steady the whole system.
"Over my many years of seeing patients in the clinic, I've realized something important. The mental health struggles modern people face, like depression, panic attacks, or sudden outbursts of anger, aren't just flaws in someone's personality. Of course, these conditions are complex, caused by a mix of genetics, environment, and various stress hormones. But in my clinical experience, a significant imbalance in serotonin is very often a major contributing factor."
"The world we live in today is heavily driven by dopamine. It constantly pushes us toward stronger stimuli and endless consumption. But that kind of hyper-stimulated happiness quickly builds a tolerance and leaves you burnt out. On the flip side, when serotonin is well-balanced, you build a steady resilience that isn't easily shaken by external pressure."
I asked him what the most effective way to keep serotonin healthy is, outside of the gut. I was expecting something complicated. The answer was a walk.
"Just leave your sunglasses behind for a bit, go for a 20 to 30 minute stroll around your neighborhood or office at a slightly brisk pace. It doesn't cost a dime, has zero side effects, and it's the best prescription available."
Dr. Lee Si-hyung
The mechanism is simple: daylight entering the eyes tells the brain to make serotonin, and steady movement keeps it flowing. In his words, the most powerful principles "always boil down to these three simple keywords: sunlight, rhythm, and connection." The second of those he describes as "the accelerator pedal that keeps the brainstem stimulated to continuously release serotonin," and it takes less than you would think: "You don't need to run until you're out of breath; the rhythmic repetition of stepping on the ground is enough to stimulate the brainstem."
And he lives all of it. Every day I was with him, he walked. As a baseball fan myself, I really enjoyed his passion for Korean baseball. It was a nice piece of common ground between us, and I spent numerous evenings watching games with him. He even keeps the TV remote far away from his chair on purpose, his own intentional inconvenience, so he has to get up to change the channel. Have you ever met a 93 year old who can do a push-up? I have now.
This part matters to me beyond the interview. A big goal for me with this project is getting more people exercising and off the chair, no matter their age. I think it is a growing problem in my generation, and I see it in some of the older people close to me too, and I want to stress how important it is. Dr. Lee's advice for people who don't exercise consistently, or only get to it once in a while, is deliberately tiny: stretch when you wake up, walk 20 minutes in daylight, and build small inconveniences into your home, like the remote across the room, or getting off the bus one stop early. You do not need a gym. You need a door.
"Medically speaking, physical movement and mental health are as deeply connected as two sides of the same coin. When you move your body, you aren't just working your muscles; you are increasing blood flow to the brain, releasing factors that support neuroplasticity, and helping your neurotransmitters, like serotonin and endorphins, find their balance. From a brain science perspective, movement is the most natural prescription we have for preventing and treating mental distress."
"Engaging in regular physical activity at a young age strengthens the prefrontal cortex, which regulates stress. Wiring your brain this way early on gives you the resilience to withstand anxiety and depression later in life."
"Don't look at exercise as a chore or a difficult homework assignment. Think of it as a pleasant time to connect with your own body, and a healthy aging process will naturally follow."
If sunlight and movement get the serotonin system going, connection is what keeps it running. Dr. Lee pointed to the Harvard Study of Adult Development, which followed people for decades and found that the strongest predictor of health and happiness late in life was not wealth or fame. It was good relationships.
"When we share a warm look with someone, or when we feel truly heard by another person, the brain releases hormones like oxytocin and serotonin, which naturally stabilize our emotions. After seeing patients for so many years, I am absolutely convinced that the ultimate medicine for a wounded mind is, in fact, human connection."
Dr. Lee Si-hyung
I asked what someone who feels isolated right now should actually do. His answer is not about forcing big friendships:
"You need to start with the smallest possible connections. It can be as simple as saying a warm 'thank you' to the barista at your local coffee shop, or even focusing your attention on a pet or tending to a plant. Relationships aren't built overnight like a brick wall. I hope you remember that a small smile shared today, or a simple greeting, is the exact starting point your brain needs to start turning your serotonin back on."
Dr. Lee Si-hyung
I carried this one across a continent. A month later in Copenhagen I interviewed Sebastian at a headspace center, where Denmark gives 12 to 25 year olds a same day, free, anonymous conversation with trained listeners, no referral needed. It goes right back to the importance of human connection that Dr. Lee kept stressing.
Related interview Sebastian at headspace, Denmark: walk-in mental health support for young peopleI brought up something I have watched grow around my own age group: drug use, and how easy access has become. Drugs like MDMA work by forcing the serotonin system to dump its supply all at once, an artificial high with a real bill. Research links that process to lasting changes in the serotonin nerve endings, which is likely why the days after can bring crushing lows.
"If you know anyone around you who is secretly struggling with this, please tell them for me: do not lose hope, and get professional help right away. The human brain has this remarkable ability called neuroplasticity. It can adapt and reshape itself. With proper medical intervention and personal commitment, the nervous system can gradually heal."
Dr. Lee Si-hyung
Something I found interesting is that his recovery advice is the same prescription as everything else: professional help first, then rebuild the ordinary days around it. Daylight, walking, eating slowly, real sleep. Not magic, raw materials. Recovery is gradual and different for everyone, but the brain's ability to recalibrate is real.
SAMHSA's National Helpline is free, confidential, and open 24/7 for substance use and mental health support in the U.S.: 1-800-662-4357. In a crisis, call or text 988. Nothing on this page is medical advice; please talk to a professional.
I am always curious about what I could be doing to live a longer, happier life, whether that means changing my routine or anything else. So at the end I asked him: if a young person wanted to build a long, happy, healthy life from scratch and you could give them one principle, what would it be? He did not hesitate.
"The strength to protect the simple rhythms of your daily routine every single day. What keeps our brain at its healthiest and most stable isn't those temporary, dopamine-fueled spikes. It is waking up at a set time each morning to catch the sun, standing on your own two feet to walk rhythmically, chewing and savoring your food slowly with gratitude, and sharing a simple smile with those around you."
Dr. Lee Si-hyung
If there is one thing I took from him, it is how much weight he puts on routine. Not grand gestures, the same small rhythms protected every single day. There is a lot of rhythm in riding a bike. Sunlight, rhythm, connection: hours of daylight, the cadence of pedaling, and strangers who become dinner companions. I am accidentally running his prescription every day of this trip. Maybe that is the realest way to fact-check him.