Jet Lag Calculator

Why the direction of travel decides how bad it gets

Jet lag is not a sleep problem, it is a clock problem. The body runs on a circadian rhythm set by light, and that rhythm is slightly longer than 24 hours, closer to 24.2. Travel changes the local clock faster than the internal one can follow.

Because the internal day runs long, it is easier to stretch it than to compress it. Flying west lengthens your day, so the body clock has to delay, which is the direction it drifts on its own. Flying east shortens your day, so the clock has to advance against its natural tendency. That asymmetry is why a 6 hour flight to Europe hurts more than a 6 hour flight to Hawaii.

The practical cost is about one day per time zone eastbound and roughly two thirds of a day per zone westbound. A 6 zone trip to Paris costs about 6 days of adjustment. The same 6 zones flown west cost about 4 days.

Two other things make eastbound worse. You lose hours instead of gaining them, so the day is shorter exactly when you need more time to recover. And most eastbound long hauls are overnight flights, which means you arrive in the morning having slept badly, then have to stay awake through an entire local day.

How many days recovery takes

The table below is the planning number to use. It assumes a healthy adult with no sleep disorder, no shift work history and no alcohol on the flight. Each of those adds days.

Older travelers usually need a little longer, and children often adjust faster than their parents, especially under 12. People who already wake at 5 am tend to handle eastbound trips better, because their clock is already early. Night owls find westbound easier for the same reason in reverse.

The number matters for scheduling. If you land in Rome on Monday and have a meeting that decides something on Wednesday, you are asking your body to perform on day 2 of a 6 day adjustment. Build in two buffer days for a trip of 6 zones or more, or arrive early enough that the meeting falls on day 4.

Worked example: New York to Paris

A 6:00 pm departure from New York, 7 hours in the air, 6 zones east. Paris is 6 hours ahead of New York in the summer.

• Departure: 6:00 pm New York time
• Arrival at home time: 1:00 am
• Arrival on the Paris clock: 7:00 am
• Body clock at arrival: 1:00 am
• Days to adjust: 6
• Target bedtime: 11:00 pm Paris time

You land at 7 in the morning with a body that believes it is 1 am. The instinct is to sleep until noon, which locks the clock onto home time for another day. Push through to a normal Paris bedtime instead, and the first night is rough but the second is better.

The fix is to force the clock forward with light rather than waiting for sleep to arrive. Get outside between 7 and 9 am Paris time for the first three mornings, even for 20 minutes, and keep the room dark in the late evening. Morning light pulls the clock earlier, which is exactly the direction an eastbound trip needs.

Melatonin helps the first few nights but not as a sleeping pill. A low dose of 0.5 mg taken 30 minutes before the target bedtime signals the body that night has started. Taking it at 3 am when you wake up does nothing useful, and taking it in the morning makes the shift worse.

Worked example: New York to Los Angeles

The same country, three zones, the other direction. A 7:00 am departure and a 6 hour flight put you in Los Angeles at 10:00 am local, which is 1:00 pm on your body clock.

• Departure: 7:00 am New York time
• Arrival on the clock: 10:00 am Pacific time
• Body clock at arrival: 1:00 pm
• Days to adjust: 2.0
• Light to seek: 5:00 to 7:00 pm Pacific time

Westbound travel feels easier because you are adding hours. You wake at 7 am Pacific time on day one, which is 10 am at home, and that is a normal hour to be awake. The cost shows up at night: you get sleepy at 9 pm Pacific because it is midnight at home, and you wake at 5 am because it is 8 am at home.

Evening light is the tool here. Stay outside or near a bright window from 5 to 7 pm for the first two evenings, and keep the morning dim. That pushes the clock later, which is the direction westbound travel needs. A 20 minute walk after dinner does more than a nap, which fragments the night that follows.

The return trip is the one that hurts. Going home from Los Angeles to New York is 3 zones east, which costs 3 days instead of 2. Travelers who plan for the outbound trip and forget the return lose the first week back at work.

Light is the strongest lever you have

Nothing resets a body clock faster than light, and the timing matters more than the amount. Bright light in the morning moves the clock earlier. Bright light in the late evening moves it later. Light at the wrong hour does the opposite of what you want.

• Flying east: seek light from 7 to 9 am for the first three mornings, and dim the room after 8 pm. Avoid a bright screen at 11 pm, which reads as afternoon light to the retina.
• Flying west: seek light from 5 to 7 pm for the first two or three evenings, and wear sunglasses in the early morning if you are outside before 9 am.
• Either direction: outdoor light is 10 to 100 times brighter than indoor light, so 20 minutes on a sidewalk beats two hours in a hotel room. Overcast daylight still works.

The clock responds to the timing of light relative to body temperature, not to the local clock alone. That is why the recommendation shifts if your sleep schedule is unusual. A shift worker flying to Europe needs a different plan than a 9 to 5 office worker, and the same 7 am advice can push the wrong way.

Caffeine is a second lever with a shorter reach. A cup of coffee at 2 pm local time delays sleep by about 40 minutes for most people, and the effect lasts 5 to 6 hours. Cut caffeine 6 hours before the target bedtime for the first three days, which for an 11 pm bedtime means nothing after 5 pm.

Melatonin: dose and timing, not sedation

Melatonin is a timing signal, not a sleeping pill. The body makes it in the evening when light fades, and a small dose taken at the right hour tells the clock that night has begun. The dose that shifts a clock is much smaller than the dose sold as a sleep aid.

The standard advice is 0.5 mg, taken 30 to 60 minutes before the target bedtime at the destination, for 3 to 5 nights. Higher doses of 3 to 5 mg are common on store shelves and are not more effective for jet lag, though they can leave you groggy. The CDC's Yellow Book chapter on jet lag lists 0.5 to 5 mg taken at local bedtime as the range studied.

Timing rules that matter more than the dose:

• Take it at the destination bedtime, even if you are not sleepy. The point is the signal, not the sedation.
• Do not take it at 3 am when you wake up. That tells the clock the night is starting again.
• Do not take it in the morning. Morning melatonin pushes the clock the wrong way for an eastbound trip.
• Stop after 5 nights. By then the light schedule should be doing the work.

Melatonin is sold as a supplement in the US, so potency varies between brands and does not have to match the label closely. If you take other medication, or you are pregnant, ask a pharmacist before adding it. And do not drive after taking it, even at 0.5 mg, until you know how you respond.

The circadian system itself is well documented by the NIH: the NIGMS fact sheet on circadian rhythms explains how the suprachiasmatic nucleus sets the daily cycle, and the American Academy of Sleep Medicine's circadian rhythm sleep disorder guide covers the clinical version of the same problem.

The flight itself: what helps before you land

Most of the damage is done in the 24 hours around the flight, not by the flight. A few choices change how the first two days go.

• Switch your watch at boarding. Start thinking in destination time the moment you sit down. If it is 2 am in Paris when you board, treat the flight as a night and try to sleep.
• Sleep on the destination schedule. On an overnight eastbound flight, sleep in the first half and stay awake for the last 3 hours so you land ready for a local day.
• Drink water, skip alcohol. Cabin air is dry, and alcohol fragments sleep in the second half of the night, which is the part that matters most for the clock.
• Eat on destination time. Meal timing is a weaker signal than light but not a zero. A meal at a local breakfast hour helps, even if it is small.
• Move on arrival. A 20 minute walk in daylight on landing does more than a nap. If you must nap, keep it under 30 minutes and before 3 pm local.

The first night is the one to protect. Get into bed at a normal local hour, keep the room dark and cool, and accept that you may wake at 4 am. If you wake early, stay in bed in the dark until 6 am, then get up and go outside. Turning on a bright screen at 4 am tells the clock to stay where it is.

Three days of discipline usually covers a 6 zone trip. The travelers who suffer for a week are the ones who nap at 2 pm, eat dinner at 9 pm and leave the curtains open.

Route planning and the return trip

The zones you cross are set by geography, and the direction is set by which way you fly. That makes some routes predictable enough to plan around.

Two planning moves are worth the money. The first is a daytime eastbound flight when you can get one, because arriving in the evening with a few hours of local daylight behind you sets up a normal bedtime. The second is a stopover on long eastbound trips: breaking a 9 zone trip into two 4 and 5 zone legs lets you sleep in a real bed and halves the adjustment on each leg.

The return trip is usually the harder one for Americans going to Europe, because coming home is eastbound in the wrong sense: you fly west to get home, which is easier, but you also lose the vacation buffer. Going to Asia from the US west coast is the reverse, with a 9 or 10 zone eastbound leg that takes over a week to settle.

Recovery times here are averages for healthy adults, not medical advice. Talk to a doctor before using melatonin if you take prescription medication, and see a sleep specialist if jet lag lasts more than two weeks.