Flying east is harder, and a 24.2-hour body clock is the reason

Fly Los Angeles to London and you’ll know about it for days. Fly London to Los Angeles, and it’s mostly an early night and a 5am wake-up. Same eight time zones. Very different week.

That gap has a physical cause. It sits in how your internal clock is built, and once you see it, most jet lag advice starts to make sense (and some of it stops).

Why is jet lag worse flying east?

Your body clock doesn’t run on exactly 24 hours. The master clock in your brain, the suprachiasmatic nucleus, has a natural period of about 24.2 hours S1. Left alone, it drifts a little later every day. Daylight is what pulls it back into line.

That small surplus matters a lot when you travel. Because the clock runs long, it has “a greater propensity to delay than to advance” S1. Pushing it later is going with the grain. Pulling it earlier is going against it.

Now apply that to a map:

• Flying west, local time is behind your body. You need to stay up later and sleep later. That’s a delay, the easy direction.
• Flying east, local time is ahead of your body. You need to fall asleep and wake up earlier than your clock wants. That’s an advance, the hard direction.

Real-world data backs this up. In an analysis of 64,847 trips from 57,240 Oura ring users, covering nearly 1.5 million sleep periods, sleep disruption “was more severe with eastward travel and across more time zones” S2. On shorter hops of up to three time zones, the shift in sleep timing was bigger after eastward trips than westward ones S2.

East vs west at a glance

	Flying east	Flying west
What your clock has to do	Move earlier (advance)	Move later (delay)
Works with the clock’s natural drift?	No	Yes S1
First evenings	Hard to fall asleep at local bedtime S1	Sleepy early in the evening S1
First mornings	Groggy late morning and early afternoon S1	Waking before your alarm S1
Light that helps	Light after your body’s low point S1	Light before your body’s low point S1
Sleep disruption in the Oura data	More severe S2	Less severe S2

What does jet lag feel like after an eastward flight?

The 2019 review in Frontiers in Physiology walks through an 8-hour eastward flight from Los Angeles to London. Straight after landing, your body is still on Los Angeles time, so the low point of your daily cycle lands around midday in London. The result: you “feel sleepy in the late morning and early afternoon” and “have difficulty falling asleep at … normal bedtime in the evening” S1.

That’s the classic eastbound pattern. You lie awake at 11pm London time because it’s 3pm for your body. Then the alarm goes off in what your body thinks is the middle of the night.

And after a westward flight?

Reverse the trip, London to Los Angeles, and the body’s low point initially falls around 8pm local time. You’ll “feel sleepy in the evening” and struggle to stay asleep until your normal wake time S1.

It’s unpleasant, but it’s the easier fix. Staying up a bit later each evening works with your clock’s natural drift instead of fighting it.

Westward isn’t free, though. The Oura data found that for people with later chronotypes (night owls), wake time after westward travel stayed earlier than their usual baseline, while earlier chronotypes returned to baseline S2. After eastward trips, it was the early birds whose wake time stayed later than baseline S2. Your own body clock type changes which direction bites you.

Why does morning sunlight sometimes make eastbound jet lag worse?

This is where the direction story gets practical, and where a lot of well-meant advice goes wrong.

Light shifts your clock differently depending on when it hits you relative to your core body temperature minimum, the low point of your daily cycle. Light in the roughly 12 hours before that low point pushes the clock later. Light in the roughly 12 hours after it pulls the clock earlier S1. For someone who normally sleeps from 23:00 to 07:00, that low point falls at about 04:00 S1.

After a big eastward flight, your low point hasn’t moved yet. On the Los Angeles to London example, it’s sitting around midday London time S1. So bright morning light in London lands before your body’s low point. It sends a delay signal, which is exactly the wrong direction when you’re trying to advance.

The review’s advice for that trip: “avoid sunlight before midday and maximize exposure to sunlight in the 3–6 h after midday” S1. Sunglasses on the morning walk. A long lunch outside.

That’s also why generic tips like “get outside first thing” can backfire after flying east. The right time for light depends on where your body clock is today, and that point moves every day as you adjust. It’s the calculation the Delagged jet lag calculator does for your specific flight.

What if I fly halfway around the world?

At a 12-hour difference, your clock could in principle shift either way. The review notes that because of that 24.2-hour period, “it is more common to adapt by delay” S1. In other words, at the halfway point the body usually gets there by going later, not earlier.

How long does it take to recover in each direction?

Longer than most people expect, especially going east.

The review gives worked examples that assume you follow its light and melatonin timing. Take the same 9-hour shift both ways. Going west, partial adaptation comes on day 3 and complete adaptation on day 7. Going east, it’s day 5 and day 8 S1. That’s with a plan. Without one, it doesn’t get faster.

The real-world data points the same way. Sleep duration bounced back quickly, to within about 12 minutes of baseline after roughly two days S2. Sleep timing and sleep architecture are another story: after long eastward trips, they can still be off two weeks later S2.

That’s the trap. You’re sleeping enough hours by day three, so you assume you’ve recovered. Your clock hasn’t.

Without any plan, the clinical rule of thumb is about a day per time zone S6. Split by direction, the unaided clock moves about an hour a day when it has to go earlier, and about an hour and a half a day when it can go later S7. So a 9-hour eastward trip takes over a week to settle on its own. The same trip westward settles in about six days. Studies of aircrew point the same way: after large shifts, westward adaptation runs roughly 50% faster than eastward S6 S8.

Does age change which direction is worse?

Less than you might think. In the Oura data, older travelers lost less sleep duration than younger ones, but age had minimal effect on how far sleep timing shifted S2. Chronotype did show up in timing, as above: night owls’ wake times stayed off after going west, early birds’ after going east S2.

What actually helps, by direction?

The mechanism tells you where to push:

• Going east: you want light after your body’s low point and darkness before it. For the first days after a big shift, that often means avoiding bright morning light and chasing early-afternoon sun S1.
• Going west: you want light before your body’s low point, which usually means bright light in the evening and staying up a little later S1.
• Melatonin isn’t a sleeping pill here. It’s a timing signal, and it has a direction. Taken in the late afternoon or early evening, it shifts the clock earlier; taken in the morning, it shifts it later S1. We cover it in detail in melatonin for jet lag. Check with your doctor before taking it.

A lot of popular advice ignores direction entirely. We went through the most common tips one by one in jet lag hacks, tested against the science.