Why is jet lag worse when you fly east?
Your body clock runs a little longer than 24 hours, so stretching your day by flying west is easier than squeezing it by flying east.
Quick Answer
Your internal body clock doesn’t run on exactly 24 hours. A landmark Harvard study found its natural cycle averages about 24.2 hours, and daylight nudges it back in sync each day. Fly west and your day gets longer, which suits a clock that already runs a little long. Fly east and your day gets shorter, forcing the clock to run early, which it’s worse at. Research reviews estimate the body clock shifts about 1.5 hours a day after westward flights but only about 1 hour a day after eastward ones. A 2016 mathematical model from the University of Maryland reached the same conclusion and found crossing about nine time zones east especially disruptive. The timing of light exposure is what resets the clock.
By the Numbers
Key Facts
- Body clock period
- About 24.2 hours on average (Science, 1999)
- Adjustment rate
- Roughly 1.5 hours a day westbound vs. 1 hour a day eastbound
- Hardest trip
- A 2016 model found about nine time zones east the most disruptive
- Main reset signal
- Light, and especially when you see it
Why east is harder
- Your clock runs long
Left alone, your body clock cycles about every 24.2 hours, slightly longer than a day.
- West stretches the day
Flying west adds hours to your day, which a slightly slow clock handles fairly easily.
- East squeezes the day
Flying east removes hours, forcing the clock to shift earlier, the direction it resists.
- Light resets it
Daylight is the main signal your brain uses to reset the clock, and its timing matters.
Flying west vs. flying east
| Flying west | Flying east | |
|---|---|---|
| Your day becomes | Longer | Shorter |
| Your body clock must | Shift later | Shift earlier |
| Typical adjustment | About 1.5 hours a day | About 1 hour a day |
A clock that runs a little slow
A master clock in the brain sets the daily rhythm of sleep, hormones and body temperature. Measured under carefully controlled lighting, its natural cycle averages about 24.18 hours, so daylight has to nudge it back into line every day.
Why direction matters
After a westward flight the clock only has to delay, which fits its natural tendency. After an eastward flight it has to advance, and research reviews estimate it manages only about an hour a day, versus about an hour and a half heading west.
The math of jet lag
In 2016, University of Maryland physicists modeled the brain’s clock cells as a network of oscillators and found the same east-west imbalance, with eastward trips across about nine time zones the hardest to recover from.
Myth vs. Fact
MythJet lag is just being tired from a long flight.
FactTravel tiredness fades after a good sleep. Jet lag comes from your body clock being out of step with local time, and it lasts until the clock catches up.
MythFlying east or west makes no difference.
FactIt does. Most people’s clocks adjust faster after flying west than after flying east.
The Bottom Line
Your body clock naturally runs a little long, so it can stretch to fit a westward trip more easily than it can shrink for an eastward one. That’s why the same number of time zones feels worse going east.
