Jet Lag Calculator
Pick where you are flying from and to and your usual bedtime. You get how many days the adjustment takes, the bedtimes to pre-shift before you leave, and when to seek and avoid light and whether to take melatonin.
Shifting bedtime 1 hour a day before departure cuts jet lag by that many hours.
Time zones crossed
6 h east
clock jumps ahead: harder
Days to adjust
3
6 without pre-adjusting
Destination bedtime
11:00 PM
keep your usual clock bedtime, in local time
Pre-trip bedtimes
8:00 PM
by the last night before flying
Before you fly: bedtime 10:00 PM (night 1), 9:00 PM (night 2), 8:00 PM (night 3), shifting wake-up by the same amount.
Light: Seek bright light in the destination morning (after your body's temperature minimum, roughly 2-3 h before your old wake time), avoid it in the late afternoon and evening for the first few days.
Melatonin: 0.5-3 mg about 30 min before the destination bedtime for the first 3-4 nights.
On arrival: switch to local meal times immediately, stay awake until at least 9 p.m. local on the first day (a 20-minute nap is fine, longer is not), and get outside in the morning.
Days to adjust, by time zones crossed
Re-entrainment at about 1 hour a day eastward and 1.5 westward (Waterhouse et al., Lancet 2007). Pre-adjusting by a day or two subtracts directly.
| Zones | Flying east | Flying west | Example |
|---|---|---|---|
| 1 | 1 days | 1 days | Chicago ↔ New York |
| 2 | 2 days | 2 days | Denver ↔ New York |
| 3 | 3 days | 2 days | Los Angeles ↔ New York |
| 4 | 4 days | 3 days | |
| 5 | 5 days | 4 days | New York ↔ London |
| 6 | 6 days | 4 days | New York ↔ Paris |
| 7 | 7 days | 5 days | |
| 8 | 8 days | 6 days | Los Angeles ↔ London |
| 9 | 9 days | 6 days | New York ↔ Dubai |
| 10 | 10 days | 7 days | Los Angeles ↔ Paris |
| 11 | 11 days | 8 days | |
| 12 | 12 days | 8 days | New York ↔ Bangkok |
The plan, by direction
| Flying east (advance) | Flying west (delay) | |
|---|---|---|
| Before you go | Bedtime and wake 1 h earlier per day for 2-3 days; bright light on waking. | Bedtime and wake 1 h later per day for 2-3 days; bright light in the evening. |
| On the plane | Sleep if it is night at the destination; eat on destination time. | Stay awake if it is day at the destination; eat on destination time. |
| Light | Seek it in the destination morning; avoid it (sunglasses) in the late afternoon and evening for 3-4 days. | Seek it in the late afternoon and evening; avoid early-morning light for 3-4 days. |
| Melatonin | 0.5-3 mg at destination bedtime, first 3-4 nights. | Usually skip; 0.5 mg if you wake at 4 a.m. and can't get back. |
| Naps | 20 minutes max, never after 3 p.m. local. | 20 minutes max; a longer one is less harmful in this direction. |
| Caffeine | Morning only. | Afternoon is fine; it helps you stay up to the later bedtime. |
What jet lag actually is
Not tiredness from the flight, which a night's sleep fixes, but a body clock that is still keeping home time: core temperature, cortisol, melatonin and hunger all peaking at the wrong local hour. The clock resets only from cues, and light is by far the strongest -- which is why the timing rules above matter more than anything you can swallow. Eating on local time helps the peripheral clocks in the liver and gut catch up; a fasting window of 12-16 hours before the destination breakfast has some evidence for the same reason.
Frequently Asked Questions
How long does jet lag last?
Roughly one day per time zone flying east and one day per one and a half zones flying west, because the body clock runs slightly longer than 24 hours and delays more easily than it advances. Six zones east is about six days, six zones west about four. Beyond 12 zones the body takes the shorter way round the clock, so a 15-zone eastward trip adjusts like a 9-zone westward one; the calculator folds the shift the same way. Light exposure at the right times and a pre-trip shift each cut a day or two off.
Why is flying east worse?
Flying east means going to bed earlier than your body wants -- advancing the clock -- and the human circadian rhythm averages about 24.2 hours, so it naturally drifts later, not earlier. Delaying (flying west) works with that drift; advancing fights it. Most people can delay about 1.5 hours a day but advance only about 1.
Does melatonin work for jet lag?
Yes, for eastward travel, with the best evidence of any supplement: a Cochrane review found 0.5 to 5 mg taken at the destination bedtime reduced jet lag on flights crossing five or more zones. Higher doses are not better; 0.5 to 3 mg is the range. For westward travel it is mostly unnecessary. It is a timing signal, not a sleeping pill.
Should I pre-adjust before the flight?
If the trip matters and is more than four or five zones, yes: shifting bedtime and wake time by an hour a day for two or three days before departure, with light at the right time, takes those hours off the adjustment at the other end. The calculator shows the bedtimes. For a short trip of one or two nights, don't adjust at all -- stay on home time.
When should I get light, and when avoid it?
Light before your body's temperature minimum (around 2-3 hours before your usual wake time) delays the clock; light after it advances the clock. Flying east you want advance, so seek morning light at the destination and avoid evening light; flying west, the opposite. Getting it backwards on the first day pushes the clock the wrong way and can add days.
These calculators use published sleep-science guidelines and are for general guidance. They are estimates, not medical advice: persistent insomnia, loud snoring or gasping in sleep, or daytime sleepiness that affects driving are reasons to see a doctor, not to adjust a bedtime.