A few days ago, I ran into a close friend of mine. He was eager to know how things had been since I started my residency in cardiothoracic surgery.

"I've been working a lot but it's been great," I replied. "It hasn't been that long since we last caught up, right?"

He looked startled and replied: "It's been six months."

Had it really been six months? It was true, but it also felt surreal. How could six months have passed by so quickly?

Residency has exceeded my expectations in all aspects. Fulfillment, reward, arduousness, fatigue. I come home each evening feeling as if I could not possibly have done one more thing.

What I didn't anticipate, though, is how residency would affect my perception of time.

As work began to dominate my schedule, time began to fly by. Days and even weeks began to blur together.

Part of the reason it felt this way is that there was so little time to reflect on everything I was doing both at the hospital and at home.

Interacting with patients is a window into the full spectrum of human nature. I witness such a wide array of adversity and resilience among my patients and their families. These encounters also teach me a lot about myself, as they can conjure emotions unexpectedly or spark a new thought.

Unfortunately, there's little time to contemplate what I see, or to fully take time to listen to my patients' stories at work. I must continue to move quickly from one bed to another, answer the next call, run to the next emergency.

Furthermore, I found myself often surrendering any sense of control over my own schedule during the first six months. I spent the first two months on a surgery rotation, then immediately switched to the intensive-care unit. This was followed by a month in the cardiology unit, then two weeks of consecutive overnight shifts in the critical-care unit. While the constant transition was stimulating, it was also head-spinning to juggle such a wide array of responsibilities.

Even at home on those rare "golden" weekends—where I had both Saturday and Sunday off—all my spare time goes to catching up on sleep, food, friends, and exercise because by Monday morning, I had to be ready to dive back into work.

That's why it took running into my friend to realize how time slips by when I'm not fully aware of my life.

Fully immersing myself in this exhilarating work is exactly what I hoped for when I signed on for this eight-year residency. Some friends were astonished it would be such a long program; it truly didn't trouble me for this reason. The only thing I can compare it to is running a marathon without thinking of how long it's lasting. What I hope is that the next 7½ years will be just as rewarding and exhilarating as the last six months.

Ironically, though, what I am also beginning to fear now isn't how many years remain in my training, but how quickly they may pass. If six months can be gone so suddenly, so can eight years.

How will I feel at the end of those eight years? Will I have a solid understanding not only of surgical treatments and medical techniques, but also how meeting patients and caring for them has changed my perspective?  Or will I wake up one morning with plenty of knowledge, yet little awareness of how I have lived the last eight years? Will I pause to catch up and carefully choose my next step? Or keep on going in the same way?

So I have but one resolution for 2018: Carve out time to absorb and reflect on all that I am learning and doing.

This goal is partly about protecting work-life balance. Relationships that inspire me to think differently, friendships that fill me with laughter, patients who allow me to grow from their stories, journeys that expand my horizons — these are the experiences that enable me to look back and see the emotional texture of time, not just a blur.

Work-life balance may seem like an unrealistic luxury for many of us, even if you belong to programs that are beginning to acknowledge its importance.

But while we may not be able to increase free time, we can guard what we do have and fill it with meaning.  William James, the American philosopher, once said, "My experience is what I agree to pay attention to." That is something that each of us can do.

I spent this past Christmas in Philadelphia with my family, who were visiting from South Korea, and New Year's Eve in Arizona with my girlfriend. I had to fight back the urge to keep working. Instead I kept reminding myself to focus on the people I care so much about, and after a while, I relaxed.

I chose to protect these precious days, and they are the memories of 2017 I'll treasure before I began the next long stretch of work.

I know I will fall short of my resolution sometimes. Yet I know I will never succeed if I don't try now, instead of waiting until the "right" time arrives.

If we are not cognizant of the passage of time, time certainly won't be cognizant of our passing.

Jason Han, M.D., is a resident in cardiothoracic surgery in the Perelman School of Medicine at the University of Pennsylvania.