Your body clock is still at home

What jet lag actually is

Deep in your brain, just above the point where the nerves from your eyes cross, is a small cluster of cells. It's your master clock. It tells the rest of your body when to feel sleepy, when to be hungry and when to be alert.

Left alone, this clock doesn't run at exactly 24 hours. A Harvard study published in 1999 measured it at 24 hours and 11 minutes on average. Every day, light nudges it back into line with the sun.

When you land somewhere new, the sun is on local time and your clock is still on home time. It catches up, but slowly: about an hour a day going east and an hour and a half going west, according to the US Centers for Disease Control. Until it does, you're sleepy at lunch and wide awake at 3am.

Jet lag or just tired? Long flights wear anyone out: broken sleep, dry cabin air, a long day. That's travel fatigue, and a good night's sleep fixes it, even after a flight from London to Johannesburg with no time change. Jet lag usually starts at three time zones or more, and only time and light fix it.

Why flying east is harder

Because your clock runs a little longer than 24 hours, it finds it easier to run late than to run early.

Fly west, from London to New York, and your first day lasts 29 hours. You stay up a little later, which your clock is happy to do. Fly east, from New York to London, and your day is squeezed into 19. You have to fall asleep when your body thinks it's early evening.

Mathematicians at the University of Maryland modelled this in 2016. They showed that a clock running even half an hour longer than a day is enough to explain why eastward trips take noticeably longer to recover from. They also found that people whose natural rhythm is closer to 24 hours suffer less in either direction.

The same logic explains point four at the top. On a very long eastward trip, it's easier for your clock to move back many hours than forward a few. Singapore is 16 hours ahead of Los Angeles on the clock. Your body usually treats the trip as eight hours behind, and so should your plan.

Your stomach has a clock too

The clock in your brain isn't the only one. Your liver, gut and other organs keep their own time. They take their cues partly from the brain and partly from when you eat.

They reset at different speeds, so for a few days your body is out of step with itself. That's the 3am hunger and the upset stomach. A 2025 study in mice found that repeated jet lag left the liver's rhythm out of step with daylight altogether.

The fix is simple: eat at local mealtimes from the first day, even if you're not hungry, and keep the first meals light.

How long it lasts

As a rough guide, allow between two-thirds of a day and a full day for each time zone you fly east, and around half to two-thirds of a day for each zone you fly west. These are averages. Age, sleep and how much daylight you get all make a difference.

If you're away for less time than that, think about whether adjusting is worth it at all. See staying on home time, below.

How light moves your clock

There's a point in the night, usually two to four hours before you'd normally wake up, when your body temperature is at its lowest. It's the hinge of your body clock.

• Light in the hours after that point moves your clock earlier. That's what you need after flying east.
• Light in the hours before it moves your clock later. That's what you need after flying west.

After a long flight, that hinge is somewhere strange on the local clock. After New York to London, it falls at about 9am London time. Bright light at 8am, the classic "go for a walk when you land", hits the wrong side of the hinge and pushes your clock the wrong way.

So the plan is two windows each day: a time to get outside, and a time to wear sunglasses and stay in dim light. They move by about an hour a day going east and about two hours a day going west, until they reach your normal morning.

Four trips, worked out

These are examples for someone who normally wakes at around 7am, using December clocks. If you wake earlier or later, move every time by the same amount. Your own body is what counts.

Melatonin, honestly

Melatonin is the hormone your body releases in the evening to signal night. As a pill, it can help on some trips, if the dose and timing are right.

• What the evidence says. A Cochrane review of ten trials found melatonin reduced jet lag after flights across five or more time zones, especially going east. Doses from 0.5mg to 5mg worked about equally well, taken close to bedtime at your destination. Higher doses didn't work better. It's still the main review, though its search was last updated in 2008.
• Small doses work. Research at MIT in the late 1990s found that 0.3mg was enough to help people sleep, around a tenth of a typical shop pill. MIT later patented low-dose melatonin, so that work isn't entirely neutral, but the CDC's guidance also says 0.5mg to 1mg is often enough.
• Timing matters more than dose. Taken at the wrong time, especially during the day, it makes you sleepy and can slow your adjustment.
• It isn't for everyone. The Cochrane authors flag people with epilepsy and people taking warfarin. Ask a doctor or pharmacist first.
• The rules differ by country. In the UK it's prescription-only, and even the NHS is split: its jet lag page says there isn't enough evidence to recommend it, while its melatonin page says it may help with jet lag symptoms. In the US it's sold as a supplement, and the CDC notes that what's in the bottle may not match the label. Check the rules where you live and where you're going before you pack any.

Before, during and after

Before you fly

• For two or three days before an eastward trip, go to bed and get up an hour earlier each day. Before a westward trip, an hour later.
• Get your sleep in. Starting a trip short of sleep makes everything worse.

On the plane

• Set your watch to destination time when you board, and eat and sleep to it where you can.
• Go easy on alcohol. It helps you nod off but breaks up your sleep.
• Pack an eye mask. It's your most useful jet lag tool, because it lets you choose when you see light.

After you land

• Follow your light windows, not the sunrise.
• Nap if you must, for 20 to 30 minutes. Longer naps eat into your night's sleep.
• Coffee is fine during the local day. Stop about six hours before bed.
• Eat at local mealtimes.

Staying on home time

If you're away for two or three days or less, adjusting may cost more than it gives. Book meetings for when you'd be awake at home, sleep on home time where you can, and you'll come back without jet lag in either direction.

The checklist

Tick these off as you go. Your ticks are saved on this device.

• Worked out how many time zones I'm crossing, and which way my body will go
• Checked my seek-light and avoid-light hours for days one to three
• Eye mask and sunglasses in my cabin bag
• Shifted my bedtime for the last two or three nights, for a long trip
• Watch set to destination time at boarding
• Asked a doctor or pharmacist about melatonin, if I'm thinking of it
• Planned meals at local times for day one
• Short trip? Decided to stay on home time

If you want a fully personalised plan, including melatonin and caffeine timing, jet lag apps such as Timeshifter and Entrain do that. Nobody pays us to mention them.

P.S. The science here is well established, but people aren't averages. If your body clock behaves differently, or a plan worked brilliantly for you, tell us on Instagram at @travelpin.app. This is general information, not medical advice.