Is Jet Lag Worse Flying East or West?

Jet lag is usually worse when you fly east. Your body clock naturally runs on a cycle slightly longer than a day, so it finds it easier to stay up later (what a westward flight asks of it) than to fall asleep earlier (what an eastward flight demands). According to the CDC's travel guidance, the average traveler adapts by roughly an hour and a half a day after heading west, but by only about an hour a day after heading east ^[2]. The American Academy of Sleep Medicine agrees that flying east is usually the harder adjustment ^[5].

Why your clock prefers to stay up late

Deep in your brain sits a master clock that times your sleepiness, hormones and body temperature. Left to itself, with no daylight to correct it, it doesn't run at exactly 24 hours. In a landmark study that kept volunteers in carefully controlled lighting, the intrinsic period of the human clock averaged 24.18 hours, in young and older adults alike ^[1].

Picture a watch that loses about 11 minutes a day. Every morning, daylight nudges it forward to keep it on time. Now imagine a trip that needs the watch set back several hours. Easy: it was already drifting that way. Setting it forward several hours fights its natural tendency. That, in a nutshell, is the east-west difference. Westward travel lengthens your day and goes with the drift, while eastward travel shortens it and goes against it ^[2].

What flying east asks of your body

Take a flight from New York to Paris. Paris is six hours ahead, so an 11 p.m. bedtime there feels like 5 p.m. to your body. You lie awake. Then the alarm goes off at 7 a.m., which your body reads as 1 a.m. You drag yourself through the morning, perk up at the wrong time in the evening, and the whole thing repeats.

Researchers have long observed that people take longer to re-entrain, or resynchronize, after a shift that moves the clock earlier than after one that moves it later, which is why eastward travel is often the tougher trip ^[4]. It isn't universal. The Sleep Foundation reports that roughly three in four people find jet lag worse going east, and individual body clocks vary ^[6].

Same light, opposite effect

Light is the main signal that moves your clock, but its effect depends entirely on when it reaches your eyes. A detailed lab study mapped this out: bright light delivered before your core body temperature bottomed out delayed the clock, light delivered after that low point advanced it, and light right at the low point had no effect ^[3]. That low point normally falls a few hours before your habitual wake time ^[2].

Here is why that matters on an eastward trip. When you land, your low point is still on home time. Arrive in Europe early in the morning after an overnight flight from the US, and your body may not have reached its low point yet, so bright morning light can push your clock the wrong way ^[6]. On big eastward jumps the clock can even end up shifting the long way around, delaying instead of advancing. Researchers call this antidromic re-entrainment, and getting a head start on the shift before you fly can lower the risk ^[4]. To find the hours when light helps or hurts you personally, try the circadian rhythm calculator.

What happens on a westward trip

Fly from London to Los Angeles and the problem flips. At 8 p.m. local time your body thinks it is 4 a.m., so you fade early in the evening and pop awake before dawn. Unpleasant, but it is the easier direction: staying up a few hours past your body's bedtime fits its natural tendency to drift later ^[1][2]. Bright evening light, which helps push the clock later ^[2], is on your side here.

Why coming home can feel worse

"Jet lag is always worse on the way home" is a common complaint, and sometimes it is just geography. Fly west from the US to Asia and the trip home is eastward, so the return leg really is the harder one for your clock ^[2].

But if you went from the US to Europe, your flight home is westward, the easier direction. If it still feels worse, direction isn't to blame. The trip itself may be. The Sleep Foundation lists short or poor sleep before you fly among the things that raise your odds of jet lag ^[6], and the last days of a holiday or business trip are rarely restful. Heading straight back to work with no buffer day doesn't help either. For how many days each leg typically takes, see how long jet lag lasts.

Working with direction, not against it

Once you know which way your clock has to move, the tactics follow. Going east, you need an advance. In the last two or three days before you fly, go to bed and get up about an hour earlier each day ^[2]; in one study, pairing that with bright morning light produced a much bigger advance than dim light did ^[4]. At your destination, the useful light is the light that comes after your body's low point, and evening light is the light to avoid ^[2][3]. Keep caffeine to the morning and stop well before bedtime ^[2].

Going west, you need a delay, which your clock is happy to give. Shift your sleep an hour later per day for 2 to 3 days before you leave, spend early evenings outdoors or in bright rooms, and keep the light dim if you wake very early, because early-morning light can pull your clock back the wrong way ^[2]. For the full day-by-day version built around your flights, the jet lag calculator does the arithmetic for you.

Your body clock is a slightly slow watch. Setting it back comes naturally; setting it forward takes a head start and well-timed light.