You land after a long flight, it’s lunchtime, and your body insists it’s 4 am. That mismatch is jet lag, or more formally circadian desynchrony: your internal clock is still set to the time zone you left. It causes poor sleep, daytime fatigue, poor concentration, headaches and digestive upset. Crossing three or more time zones is usually enough to notice it. The good news is that the body clock responds predictably to light, and with a plan you can adjust days faster. Our jet lag calculator builds a personalised schedule for your route.
This guide is general information, not medical advice. If you have a sleep disorder, take regular medication or are pregnant, ask a doctor before using sleep aids or melatonin.
Why jet lag happens
A master clock in the brain (the suprachiasmatic nucleus) sets daily rhythms for sleep, hormones, body temperature and digestion. It is reset mainly by light hitting the eyes, with smaller effects from meals, exercise and social cues. When you fly across time zones faster than the clock can shift, every system runs on the old schedule for a while.
Travel fatigue (dehydration, cramped seats, poor sleep on board) adds to the symptoms but goes away after a good night’s sleep. Jet lag lasts longer.
East vs west
| Direction | What you need to do | How fast you adjust | Example |
|---|---|---|---|
| East (e.g. New York → London, London → Dubai, London → Tokyo) | Advance the clock: sleep and wake earlier | About 1 hour per day | London → Tokyo in winter: 9 hours ahead, often a week or more |
| West (e.g. London → New York, London → Los Angeles) | Delay the clock: sleep and wake later | About 1.5 hours per day | London → New York: 5 hours behind, often 3–4 days |
The human body clock naturally runs a little longer than 24 hours, so staying up later is easier than going to bed earlier. That is why most people find eastbound trips harder. For very large shifts (8 hours or more eastward), the body sometimes shifts the “wrong” way round the clock, which takes even longer.
The light rules
Light is the strongest tool. Its effect depends on when you get it relative to your body clock’s internal night:
- Light in the hours after your body’s lowest temperature point (roughly the last couple of hours of your usual sleep and the early morning) advances the clock, helping eastward travel.
- Light in the evening and first half of the night (by home time) delays the clock, helping westward travel.
- Light at the wrong time pushes the clock the wrong way.
Practical light plan
Flying east (e.g. London → Dubai, 4 hours ahead in UK winter, or New York → London, 5 hours ahead):
- Seek bright morning light at the destination from day two or three, and go outside in the late morning.
- On the first day after a big eastward jump, avoid very early bright light (wear sunglasses), because that early-morning light can fall in your body’s “delay” window.
- Dim screens and lights in the evening.
Flying west (e.g. London → New York):
- Get outdoor light in the afternoon and early evening.
- Avoid bright light very early in the morning for the first couple of days; keep the bedroom dark if you wake at 4 am.
Daylight is far brighter than indoor lighting, so a walk outside beats any lamp.
Shift your schedule before you go
For trips of more than a few days, start adjusting before departure:
- Eastward: go to bed and get up 1 hour earlier each day for 2–3 days before the flight, getting bright light soon after waking.
- Westward: go to bed and get up 1 hour later each day, seeking light in the evening.
Three days of pre-shifting can take three hours off the problem before you even board.
On the plane
- Set your watch and phone to destination time as soon as you board, and eat and sleep according to it where possible.
- If it’s night at the destination, sleep: use an eye mask, earplugs and avoid caffeine.
- If it’s day at the destination, stay awake, keep the window shade open if allowed and move around.
- Drink water regularly and go easy on alcohol, which worsens sleep quality.
On arrival
- Eat on local time. Meal timing helps reset clocks in the liver and gut.
- Nap carefully. If you must, keep it to 20–30 minutes and before mid-afternoon.
- Caffeine can help daytime alertness but avoid it within about six hours of local bedtime.
- Exercise outdoors in daylight combines two signals.
- Short trips (two days or less): consider staying on home time instead, scheduling important meetings when you would naturally be alert.
Melatonin and sleeping pills
- Melatonin is the hormone your body produces in the evening. Research, including a Cochrane review, found it can reduce jet lag when taken close to the target bedtime at the destination, particularly after eastward flights crossing five or more time zones. Low doses appear to work about as well as high ones for shifting the clock.
- In the UK, melatonin is a prescription-only medicine. In the US it is sold as a dietary supplement, and product strength can vary.
- Sleeping pills may improve sleep duration but do not shift the body clock, and can cause next-day grogginess.
Always check with a pharmacist or doctor, especially if you take other medicines.
Worked example: London to Tokyo
Tokyo is 9 hours ahead of London in UK winter (8 in UK summer). A typical evening departure lands in Tokyo in the late afternoon or evening the next day.
- Three days before: go to bed and wake 1 hour earlier each day (by departure, 3 hours shifted).
- On board: sleep during Tokyo’s night, stay awake during Tokyo’s day.
- Day 1 in Tokyo: stay up until about 10 pm local, then sleep.
- Days 2–4: outdoor light in the late morning and around midday, dim evenings.
- Expect lingering tiredness for about a week; schedule demanding tasks for local late morning.
Checklist
- Work out the direction and number of time zones crossed.
- Pre-shift sleep by an hour a day for 2–3 days if possible.
- Switch to destination time on boarding.
- Use light deliberately: morning for east, evening for west.
- Eat, nap and drink caffeine on local time.
- Ask a professional before using melatonin or sleeping pills.
To understand the time differences themselves, read our time zones guide or use the time zone converter.