What causes jet lag
Your body runs on a roughly 24-hour internal clock – the circadian rhythm – that controls sleep, alertness, hunger and body temperature. When you cross several time zones quickly, local daylight and meal times no longer match that clock. The result is jet lag: poor sleep at night, sleepiness during the day, difficulty concentrating, and digestive upset. It lasts until your internal clock catches up with local time.
East vs west
Most people's natural clock runs slightly longer than 24 hours, so it's easier to stay up later than to fall asleep earlier. Flying west lengthens your day and is usually easier; flying east shortens it and is harder. A common rule of thumb is that the body adjusts about one time zone per day after flying east and about one and a half per day after flying west, though individuals vary.
Recovery days ≈ time zones crossed eastward × 1 · westward × 0.67
Examples
| Route | Difference | Direction | Typical recovery |
|---|
| New York → London | +5 h | East | about 5 days |
| London → New York | −5 h | West | about 3 days |
| Los Angeles → Tokyo | +16 h (≈ −8 h) | Treated as west | about 5 – 6 days |
| London → Dubai | +3 – 4 h | East | about 3 – 4 days |
Light is the strongest signal
Daylight is the main cue that resets the body clock. After flying east, morning light helps you advance your clock (wake earlier); evening light pushes it the wrong way. After flying west, late-afternoon and evening light helps you stay up later. Sunglasses at the wrong time of day can be as useful as daylight at the right time.
Practical tips
- Start shifting your sleep by an hour a day for a few days before departure.
- Set your watch and phone to destination time when you board.
- Sleep on the plane only during the destination's night.
- Drink water, go easy on alcohol and limit caffeine after midday local time.
- Eat meals at local times – food timing also nudges the body clock.
- Short naps (under 30 minutes) are fine; long afternoon naps delay adjustment.
Gear that helps
A comfortable eye mask, earplugs or noise-cancelling headphones, and a watch with a second time zone (GMT function) make it easier to sleep on board and keep track of home time. Some travelers use light-therapy glasses or apps that schedule light exposure. Ask a healthcare professional before trying melatonin or sleep medication.
General information only – not medical advice.
Frequently asked questions
How long does jet lag last?
Roughly one day per time zone after flying east and about two-thirds of a day per zone after flying west, varying between individuals.
Is jet lag worse flying east or west?
Flying east is usually harder, because it requires falling asleep earlier than your body is used to.
How can I avoid jet lag?
Shift your sleep gradually before the trip, get daylight at the right time at your destination and adopt local meal and sleep times immediately.
Should I adjust to local time on a short trip?
For trips of two or three days, many travelers stay on home time as much as possible instead of adjusting.
Does melatonin help jet lag?
Some studies show benefits, but dosing and timing matter. Ask a doctor or pharmacist before using it.
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