Why Jet Lag Has More to Do With Light Than With Flying

For a long time, I thought jet lag was just part of the deal. You cross half the planet, you suffer a bit, and eventually your body catches up. Nothing more to think about.

For me, traveling from Japan to Europe was usually fine. A little tiring, yes, but manageable. Traveling back from Europe to Japan, however, was often a completely different story. Sometimes the jet lag would stay with me for days. Sometimes it would linger for weeks and turn even simple days into a strange mix of tiredness and restlessness.

Then, on a recent trip back from the Netherlands to Japan, something unexpected happened. I arrived almost completely fine. No long recovery. No weeks of broken sleep. Just a normal rhythm, almost as if nothing had happened.

Same distance. Same destination. Very different result.

That made me curious. Not just about my own experience, but about jet lag itself. What is it really? And why does it sometimes feel so brutal and sometimes almost nonexistent?

It turns out the answer has surprisingly little to do with flying.

Jet lag is a clock problem.

Inside the brain, there is a biological timing system called the circadian rhythm. It quietly regulates when we feel sleepy and awake, how hormones are released, how digestion works, and how clear or foggy the mind feels during the day.

This clock does not move just because we do.

When we cross many time zones in a short time, the body is still living in the old time zone, even though everything around us has already shifted. That gap between internal time and local time is what we experience as jet lag.

Many people think sleep is what sets this clock. In reality, light is the strongest signal. Morning light tells the brain that the day has begun. Darkness in the evening tells it to prepare for rest. Because of this, light at the wrong time can gently but persistently push the clock in the wrong direction and delay adaptation by days.

The importance of this system is so fundamental that research on biological clocks was awarded the Nobel Prize in Medicine in 2017.

There is also a quiet asymmetry in how we adapt to time changes.

Traveling west makes the day longer. Traveling east makes the day shorter. Most of us find it easier to stay up later than to fall asleep earlier. Because of this, shifting the body clock later is easier than shifting it earlier.

This is why westward travel often feels easier, and eastward travel often feels heavier.

On average, the body adapts about one time zone per day when traveling west, but only about one time zone every one and a half to two days when traveling east. A seven or eight hour eastward shift can therefore easily take ten to fourteen days to fully settle. And when light exposure, sleep timing, and daily habits are not aligned, it can take even longer.

This also explains why some trips feel much worse than others.

Long and stressful journeys, fragmented sleep, badly timed light exposure, and long daytime naps all make it harder for the internal clock to find a new rhythm.

The good news is that this process is not completely out of our hands.

Shifting sleep times slightly before departure already helps. After arrival, getting daylight in the morning and keeping evenings dim helps even more. Short naps can be useful. Long naps usually just confuse the body further. And gently anchoring meals, sleep, and daily activity to local time gives the nervous system clear and repeated signals about where it is.

Melatonin can help some people, but it works best as a support, not as a replacement for light and routine.

The simple truth is this.

Jet lag is not really about distance.

It is about how quickly the brain can be convinced that morning and evening have moved.

Once this is understood, jet lag stops being a mysterious curse and starts to look more like what it actually is. A slow, biological process of retuning the body to a new rhythm. One that we cannot force, but that we can support with a bit of understanding and patience.

If you want to explore this topic a little deeper, here are some reliable and accessible starting points:

Nobel Prize in Physiology or Medicine 2017 Awarded for discoveries of the molecular mechanisms controlling the circadian rhythm.
National Institutes of Health (NIH), National Institute of General Medical Sciences Educational resources on circadian rhythms and biological clocks.
Sleep Foundation Clear articles on circadian rhythm, jet lag, light exposure, and sleep timing.
Cleveland Clinic and Mayo Clinic Medical overviews on jet lag, its causes, symptoms, and treatment strategies.
Stanford Sleep Medicine Center Resources on circadian rhythm disorders and light-based clock shifting.
Matthew Walker, “Why We Sleep” A popular science overview of sleep and circadian rhythms.
Satchin Panda, “The Circadian Code” A book focused on how light, timing, and daily rhythms regulate the body.