Why flying east is harder (and what light has to do with it)

Ask anyone who flies often and they’ll tell you: New York to London hurts more than London to New York. That isn’t just anecdote, and the reason is built into your biology.

Your clock runs slightly long

Left without daylight, the human circadian clock runs a bit longer than 24 hours, on average a few minutes over. Every day, morning light nudges it back into line.

That small bias has a big consequence. Delaying the clock (going to bed later, as when flying west) goes with the grain. Advancing it (going to bed earlier, as when flying east) goes against it. A common rule of thumb: you recover from about one time zone per day going east, and closer to one and a half per day going west.

The light switch

The most powerful signal to your clock is light hitting your eyes. What it does depends on when:

• Light in the evening and first half of the night pushes your clock later.
• Light in the late night and morning pulls it earlier.

The turning point is your core body temperature minimum, which sits about two hours before you’d naturally wake. Light just before it delays you; light just after it advances you.

The eastbound trap

Here’s the catch on a long eastward flight. Say you fly from New York to Paris and land at 8 a.m. local time. To your body, it’s 2 a.m., before your temperature minimum. The bright morning sun in the arrival hall falls on the delaying side of the switch and pushes your clock further in the wrong direction.

The fix sounds backwards: on the first day, wear sunglasses in the morning and save bright light for late morning and early afternoon, when it lands on the advancing side. Each day the good window moves earlier, until it lines up with the local morning.

What to actually do

1. Start early if you can. Shifting bedtime an hour a day for two or three days before you fly takes a big bite out of the problem.
2. Follow the light windows, not the clock. The jet lag calculator works out where your temperature minimum sits each day and tells you when to seek light and when to hide from it.
3. Sleep on local time. Naps under 30 minutes are fine; long afternoon naps anchor you to home.
4. Consider melatonin going east. Low doses (0.5-3 mg) at local bedtime have decent evidence behind them. Ask a pharmacist if you take other medication.

Flying west, the plan is gentler: stay up until a reasonable local bedtime, get evening light, and you’ll usually be fine in a couple of days.

Sources

• Sack RL. Jet lag. New England Journal of Medicine. 2010;362(5):440-447.
• Eastman CI, Burgess HJ. How to travel the world without jet lag. Sleep Medicine Clinics. 2009;4(2):241-255.
• Czeisler CA, et al. Stability, precision, and near-24-hour period of the human circadian pacemaker. Science. 1999;284(5423):2177-2181.
• Herxheimer A, Petrie KJ. Melatonin for the prevention and treatment of jet lag. Cochrane Database of Systematic Reviews. 2002;(2):CD001520.