Jet lag is not tiredness from a long flight. You can sleep well on a twelve-hour trip north to south and arrive fine, then feel wrecked after a seven-hour hop across the Atlantic. The difference is time zones, not hours in a seat. Your body runs on an internal clock, and when you move faster than that clock can follow, it stays on home time while the world around you has moved on.
This guide explains what is going on, why direction matters, which remedies have real evidence behind them, and gives a practical plan you can adapt. It is general information, not medical advice: if you have a health condition or take regular medication, talk to a doctor or pharmacist before trying melatonin or sleep aids.
What jet lag actually is
Almost every cell in your body keeps time, but they are synchronised by a master clock in the brain, a small region of the hypothalamus called the suprachiasmatic nucleus. That clock drives the daily rise and fall of body temperature, hormones such as melatonin and cortisol, alertness and digestion.
The clock is not perfectly 24 hours. A well-known study in Science found the human circadian period averages about 24.18 hours when people are kept away from normal light cues. Every day, light (especially morning light) nudges it back into line with the sun.
The CDC Yellow Book, the US travel medicine reference, defines jet lag disorder as a sleep problem caused by rapid travel across at least two time zones, with insomnia or daytime sleepiness and impaired functioning within a day or two of arrival. Symptoms include poor sleep, foggy thinking, daytime sleepiness, general malaise and stomach trouble.
Why flying east is harder
Flying east means the day gets shorter: you need to go to bed and wake up earlier than your body expects. Flying west stretches the day. Because the internal clock runs slightly longer than 24 hours, it is naturally easier to delay than to advance.
The CDC gives an average adaptation rate of about 1.5 hours per day after westward travel and about 1 hour per day after eastward travel. In rough terms:
| Trip | Time difference | Direction | Rough days to adapt (CDC averages) |
|---|---|---|---|
| Rome to New York | 6 hours | West | About 4 |
| New York to Rome | 6 hours | East | About 6 |
| London to Singapore | 7 or 8 hours | East | About 7 to 8 |
| Paris to Los Angeles | 9 hours | West | About 6 |
These are averages, not promises. Some people adapt faster, and the steps below can speed things up. There is also a twist for very large eastward shifts: the body sometimes adjusts by delaying instead, going the long way around the clock. Researchers call this antidromic re-entrainment, and a German lab study that simulated a nine-hour eastward flight observed it unless the shift was split across two days. A mathematical model from the University of Maryland, published in the journal Chaos, estimated that recovering from a nine-time-zone trip takes about 12 days going east and about 8 going west.
The flight time itself also differs by direction, which has nothing to do with your body clock and everything to do with the jet stream. Our post on why westbound flights take longer explains why the trip home can be an hour longer than the trip out.
Light: the strongest tool you have
Light is the main signal that resets your clock, and timing matters more than quantity. The CDC explains it around the low point of your daily cycle (the circadian nadir, close to when your body temperature is lowest), which it places roughly 2 to 4 hours before your usual wake time:
- Light after the low point (your body's morning) advances the clock. You want this after flying east.
- Light before the low point (your body's evening and night) delays the clock. You want this after flying west.
So for an eastward trip the CDC suggests seeking bright light in the early morning and avoiding light in the evening. For a westward trip, avoid bright early morning light (curtains, sunglasses) and stay up until local bedtime, getting light in the afternoon and evening.
The catch on big eastward trips: the destination's early morning may still be your body's middle of the night. Bright light then pushes your clock the wrong way. If you have crossed many time zones going east, it can help to wear sunglasses for the first couple of local mornings and get your daylight from late morning onwards, moving it earlier each day. This is what jet lag calculator apps do: they work out when your body's morning really is.
Daylight outdoors is far brighter than indoor lighting, even on a cloudy day. A walk outside at the right time does more than any number of hotel lamps.
Melatonin: useful, but less than it is sold as
Melatonin is the hormone your body releases in the evening as darkness signals night. Taken as a pill, it acts in two ways: a mild sleepiness effect, and a clock-shifting effect that depends on timing.
What the evidence says:
- A 2002 Cochrane review, summarised in American Family Physician, found that in nine of ten trials melatonin reduced jet lag after flights across five or more time zones, especially going east, and that occasional short-term use appeared safe. It flagged possible problems for people with epilepsy and people taking warfarin.
- Later reviewers have been more cautious. A 2020 review in the Drug and Therapeutics Bulletin, reported by the Pharmaceutical Journal, pointed out that the trials were small, mostly from 1986 to 2005, and often did not report the melatonin used, and concluded there is "limited published evidence" of benefit.
The CDC's practical guidance is that low doses of 0.5 to 1 mg are often enough to shift the clock, and that doses above 5 mg are not recommended because extra melatonin can linger into the wrong time of day. Timing matters more than dose: taken in your body's early evening it advances the clock (useful after flying east; the CDC's example schedule has it about 90 minutes before local bedtime), and taken in your body's morning it delays it. The CDC also notes it works less well when taken while your own melatonin is already high, roughly midnight to 5 a.m. on body time.
Two practical warnings. In the US, melatonin is sold as a supplement and is not FDA regulated, so the CDC notes the actual content may differ from the label. Rules differ by country: in the UK it is a prescription medicine, and the rules vary across Europe. And if you take other medication, ask a pharmacist first: interaction data are limited.
Caffeine, alcohol, naps and sleeping pills
Caffeine does not shift your clock much, but it helps you stay functional. Its half-life is about five hours, so the CDC suggests using it in the destination's daytime (its example is 200 mg every four hours as needed) and stopping six hours before bedtime.
Alcohol helps you fall asleep but fragments sleep later in the night. Combined with the dry cabin air, it tends to make the first nights worse. Go easy on it in the air and on the first evening.
Naps of 20 to 30 minutes can rescue an afternoon. Long naps make it harder to sleep at night. If you arrive in the morning after an overnight flight, a short nap is fine; a four-hour nap is how you end up awake at 3 a.m.
Sleeping pills are listed by the CDC as an option, but they do not fix the underlying clock problem, and they come with side effects. The CDC specifically advises against long-acting sedatives during the flight, and says antihistamines should be avoided because they can worsen cognitive impairment. These are a conversation with a doctor, not something to improvise.
A practical plan
Here is a simple version for a trip of five or more time zones. Adjust for direction.
Three days before
- Shift your bedtime and wake time by about an hour per day toward the destination: earlier if going east, later if going west. The CDC suggests doing this for two to three days before departure, where your schedule allows.
- Get light at the times that match the new schedule.
On the flight
- Set your watch and phone to destination time as soon as you board, and think in that time.
- If it is night at the destination, try to sleep: eye mask, earplugs, minimal screens. If it is day there, stay awake as much as you reasonably can.
- Drink water, keep alcohol and caffeine light.
A nonstop flight helps more than people expect. You arrive earlier, with one block of disrupted sleep instead of two, and you can plan the light schedule around a single arrival time. Our comparison of nonstop vs connecting flights covers the other trade-offs.
First days at the destination
- After flying east: get outdoor light from late morning, avoid bright light in the evening, go to bed at local time even if you are not sleepy. If you use melatonin, a low dose an hour or two before local bedtime is the usual approach.
- After flying west: get outdoor light in the afternoon and early evening, stay up until local bedtime, and protect yourself from very early morning light if you wake at 4 a.m.
- Both directions: eat at local mealtimes, keep naps short, and schedule anything important for the time of day when you will be sharpest (usually your home-time afternoon).
For short trips
If you are away for less than about two days, the guidance the CDC cites says not to adapt at all: stay on home time for sleep, meals and activity, book meetings for the hours that suit your body, and avoid the light that would shift you. On a three- or four-day trip it is a judgement call. The CDC notes that frequent travellers face cumulative effects of repeated sleep loss, so the less you shift back and forth, the better.
Frequently asked questions
How long does jet lag last?
According to the CDC's averages, about one day per hour of time difference after flying east and a little less after flying west. A six-hour eastward shift typically takes around six days to fully settle, though the worst symptoms usually ease sooner.
Does staying hydrated cure jet lag?
No. Dehydration from dry cabin air can make you feel worse, so drinking water helps you feel better, but it does not move your body clock. Light and timing do that.
Is it better to arrive in the morning or the evening?
For eastward trips, a morning arrival after an overnight flight is common and workable if you get daylight and avoid a long nap. For westward trips, an afternoon or evening arrival makes it easy to stay up until local bedtime. The best arrival time depends on direction and how many zones you cross.
Do children get jet lag?
Yes, but they often adapt reasonably quickly. Keep them on local mealtimes and get them outside in daylight. Do not give children melatonin or sleep medication without a doctor's advice.
Does flying north to south cause jet lag?
Not if you stay in roughly the same time zone. A long flight from Europe to South Africa can leave you tired, but not jet lagged in the circadian sense. Long flights such as those in our list of the longest nonstop flights usually combine both.