How the jet lag calculator works
Jet lag happens because your body clock stays on home time after you land. The calculator compares the time zones of your two cities on your travel dates, including daylight saving time, and works out how many hours your body clock has to move and in which direction.
It then builds a plan around one marker: the coldest point of your body temperature, which falls about 3 hours before your usual wake time. Light in the hours after that point moves your clock earlier. Light in the hours before it moves your clock later. The plan tells you when to sleep, when to get outside and when to wear sunglasses, day by day, until your body clock matches local time.
The method follows the jet lag research of Charmane Eastman and Helen Burgess at Rush University Medical Center. For the full method, with examples for flights to Europe, Asia and Australia, read how to beat jet lag going east or west.
Jet lag recovery time formula
Jet lag recovery time is the number of hours your body clock has to shift, divided by how many hours it can shift per day:
Recovery days = time zones crossed ÷ hours shifted per day
Without any plan, the body clock shifts about 1 hour per day after flying east and about 1.5 hours per day after flying west. That is where the common rule of "one day per time zone" comes from. It fits eastbound trips. Westbound trips take about a third less time.
With well-timed light and sleep, the clock shifts about 1.5 hours per day going east and 2 hours per day going west. Each day you shift before the flight takes about 1 more hour off.
| Time zones | East, no plan | East, with plan | West, no plan | West, with plan |
|---|---|---|---|---|
| 3 | 4 days | 2 days | 2 days | 2 days |
| 5 | 6 days | 4 days | 4 days | 3 days |
| 6 | 7 days | 4 days | 4 days | 3 days |
| 8 | 9 days | 6 days | 6 days | 4 days |
On very long eastward trips the body clock often shifts the other way. After London to Sydney (11 hours ahead), it is faster to move the clock 13 hours later than 11 hours earlier, because moving later goes faster per day. New York to Tokyo (13 or 14 hours ahead) works the same way: the clock moves 10 or 11 hours later. The calculator picks whichever direction gets you adjusted sooner.
When to sleep after a long flight
After flying east, go to bed at a normal local bedtime, even if you are not sleepy. Do not sleep in past 2 hours after your usual wake time. Morning light can push your clock the wrong way for the first day or two, so the calculator may tell you to avoid bright light early and get outside later in the morning.
After flying west, stay awake until at least 9 pm local time. Get light in the late afternoon and evening. If you wake very early, stay in a dark room until your planned wake time.
On the plane, set your watch to destination time when you board. Sleep only if it is night at your destination. On a short trip of 1 or 2 nights, stay on home time instead: the calculator offers that option.
Naps under 30 minutes before 3 pm help on the first day. Longer naps make the next night harder. Coffee in the morning is fine. Stop about 8 hours before bedtime.
Jet lag calculator on Reddit: what travelers say
On r/travel, the jet lag threads repeat the same points:
- Shift before you go. Going to bed an hour earlier each night before an eastbound trip is the most common tip that people say works.
- Timeshifter splits opinion. Fans say it works if you follow the advice exactly. Others found the pre-trip schedule too hard to keep. It gives one free plan, then needs a subscription.
- Nights 2 to 4 are the hard ones going east. Many travelers sleep well the first night out of exhaustion, then wake at 3 am for several nights. That matches the 1 hour per day the body clock shifts eastward.
- West is easier. Most people feel normal within a day or two after flying west.
- Go on local time at boarding. Eat and sleep by destination time from the start of the trip.
This calculator gives the same kind of light and sleep plan as the paid apps, for free, in your browser, with no account.
Frequently asked questions
How long does jet lag last?
Without a plan, jet lag lasts about 1 day per time zone after flying east and about 2 days for every 3 time zones after flying west. A 6-hour eastward trip, such as New York to Paris, takes about 6 to 7 days. With timed light and sleep, the same trip takes about 4 days.
Is jet lag worse going east or west?
East. Flying east makes your day shorter, and the body clock moves earlier more slowly than it moves later. Most people adjust about a third faster after flying west.
What is the jet lag recovery time formula?
Recovery days equal the time zones crossed divided by the hours your body clock shifts per day. Use 1 hour per day for east and 1.5 hours per day for west without a plan, or 1.5 and 2 hours with a light and sleep plan.
When should I sleep to avoid jet lag?
Sleep at your usual bedtime in local time from the first night, and sleep on the plane only if it is night at your destination. Enter your flight in the calculator above to see your sleep times for each day.
Should I stay awake when I land?
Yes, until a normal local bedtime, with at most one nap of under 30 minutes before 3 pm. Going to bed in the afternoon keeps your body on home time.
Does melatonin help with jet lag?
Yes, mainly after flying east across 5 or more time zones. A low dose of 0.5 to 3 mg works about as well as a large one, and the timing matters more than the dose. Take it at your new local bedtime for the first few nights. Ask a doctor first if you take other medicines.
Can you get over jet lag in one day?
Not for trips of more than 3 time zones. The body clock shifts at most about 2 hours per day, so a 6-hour change takes at least 3 days even with a good plan. Shifting your sleep for a few days before the flight is the only way to arrive with less jet lag.
Is this jet lag calculator free?
Yes. It runs in your browser, needs no account and stores nothing. You can add the plan to your calendar or print it.
Sources
- Eastman CI, Burgess HJ. How to travel the world without jet lag. Sleep Medicine Clinics, 2009.
- CDC Yellow Book. Jet lag disorder.
- Herxheimer A, Petrie KJ. Melatonin for the prevention and treatment of jet lag. Cochrane Database of Systematic Reviews, 2002.
- Sleep Foundation. Jet lag: symptoms, causes and treatment.
Related: how to sleep on a plane in economy and the packing list generator.