Why jet lag is worse flying east: 6 days vs 4 for the same 6 h

The flight from Berlin to New York always feels like a gift. You land in the afternoon, you're tired at 9 pm local time, you sleep like a log and wake up at 5 am feeling weirdly productive. Then you fly home, and for the better part of a week you are a person who stares at the kettle without remembering why.

So why is jet lag worse flying east? Because your body clock is much happier being pushed later than being pushed earlier. Flying west asks it to stay up a bit longer, which it does almost for fun. Flying east asks it to go to bed when it thinks it's early evening, and it refuses. I built the Jet Lag Planner partly to put numbers on that feeling, and the numbers are pretty clear.

The short answer: your inner clock likes to run late

For most people the internal clock runs a little longer than 24 hours. Left alone in a cave, you'd drift a bit later every day. That's why staying up late on a Friday is easy and getting up early on a Monday is not.

A westward flight works with that drift: New York is 6 hours behind Berlin, so you simply have a very long day and go to bed "late". An eastward flight works against it: Berlin is 6 hours ahead of New York, so at 11 pm in Berlin your body still thinks it's 5 pm and wants dinner, not sleep.

The planner turns this into a simple planning rule: after arrival, the body clock catches up about 1.5 hours per day after flying west and only about 1 hour per day after flying east. That's a rule of thumb, not a law of nature, but it explains a lot.

Same 6 hours, two very different weeks

Here's the same trip in both directions, with no preparation at all (0 days of pre-adjustment). Berlin and New York use the default sleep times of 11 pm to 7 am, London and Tokyo the tool's example of a night owl sleeping 23:30 to 07:30:

Same number of hours, and the eastbound version costs two extra days in both pairs. On a one-week business trip, that's the difference between "fine by Wednesday" and "fine by the flight home".

The light advice is where it gets really interesting, because it flips with the direction:

• Berlin → New York, first evening: seek bright light 19:30–22:30. Take the evening walk.
• New York → Berlin, first morning: avoid bright light 07:30–10:30 and seek it 10:30–13:30.

That second one surprised me. My instinct after an overnight flight east was always "get into the morning sun!" But on day one your body is still running on New York time, and early light lands on the wrong side of its night. It nudges your clock later, which is exactly the wrong way. Sunglasses for the taxi, then a long lunch walk. The next days, the windows slide earlier by an hour: 09:30–12:30 on day two, 08:30–11:30 on day three, and so on.

Getting a head start before you fly

The planner's real trick is the Days to pre-adjust before departure field. It moves your sleep 1 hour per day towards destination time before you leave, as the CDC Yellow Book suggests for the 2–3 days before a trip.

With 3 days of prep:

• New York → Berlin: bedtime moves from 23:00 to 22:00, 21:00 and finally 20:00 (wake at 04:00, sorry). Days to adapt after arrival drop from 6 to 3.
• Berlin → New York: bedtime moves to 00:00, 01:00 and 02:00. Days to adapt drop from 4 to 2.
• London → Tokyo: from 8 days down to 5.

Two honest observations. First, going to bed at 20:00 for a few evenings is easy to type and hard to do if you have kids, a job or friends. Even one day of prep helps, so don't skip it just because three feels impossible. Second, preparing for a westward trip is the fun one: "stay up an hour later" is the easiest medical advice in the world.

And a little confession. The London → Tokyo example in the tool is labelled "9 h". The tool says +8 h. It's right and my label is wrong: on 2 April the UK is already on summer time, Japan never changes its clocks, so the gap shrinks to 8 hours. The same thing happens this week: on 30 October 2026, Berlin → New York is only −5 h, because Europe switched back to winter time a week before the US. This is exactly why the planner looks up the real offset for your travel date instead of trusting a number someone (me) typed into a label.

How I fill in the fields for a trip east

For a New York → Berlin trip, this is what I'd enter:

1. Home time zone: America/New_York, Destination: Europe/Berlin.
2. Departure date and Arrival date (local): overnight flights east usually land the next day, so set the arrival date, or the plan starts a day off.
3. Usual bedtime / wake time: what you actually do, not what you wish you did.
4. Days to pre-adjust: 2 or 3 if you can, 0 if life says no.
5. Chronotype: night owls get their body temperature minimum placed a bit later, which shifts the light windows.

Then hit Add the plan to your calendar (.ics). You get calendar events for "seek bright light", "avoid bright light", "last caffeine" (6 hours before bed, so 17:00 for an 11 pm bedtime) and bedtime. Your phone nags you so your tired brain doesn't have to remember.

Be honest: what the planner can't know

• It's planning guidance, not medicine. Nothing here replaces a doctor, especially if you're thinking about melatonin or sleeping pills.
• The adaptation rates are averages. 1 hour per day east, 1.5 west. Some people bounce back faster, some much slower, and age plays a part.
• It assumes you actually sleep at the planned times. Naps, the airline's free wine and the 3-hour film you watched instead of sleeping aren't modelled.
• It always takes the shorter way round the clock. For very large shifts (10 hours or more) some travellers adapt better the "long" way; the tool warns you, but it won't plan that for you.
• Short trips are different. If you're away for less than about 2 days, staying on home time is often smarter than adapting at all.

If you're flying east soon, open the Jet Lag Planner, click the London → Tokyo (eastward) example, and then set Days to pre-adjust to 0 and back to 3. Watching "8 days" turn into "5 days" is the most convincing argument for an early night I know.