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Your body clock shifts about 1 hour per day when you fly east and about 1.5 hours per day when you fly west (rates from Eastman and Burgess, 2009). Dividing the time zones crossed by that daily rate, minus any head start you banked before flying, gives the estimated recovery days. Your body-temperature minimum sits about 3 hours before your usual wake time: seeking bright light in the 4 hours after it pulls your clock earlier (eastbound), while light in the 4 hours before it pushes your clock later (westbound).
Jet lag is your internal clock disagreeing with the local clock. Research by Eastman and Burgess (2009) found the clock can move about 1 hour per day eastward and about 1.5 hours per day westward when light is timed well. So a 6-zone trip east needs roughly 6 days, while the same trip west needs about 4 days.
If you start shifting before you fly, you get credit: each pre-flight shift day knocks about 0.7 zones off the bill eastbound. That is why a 3-day head start on a New York to London trip can cut recovery from 6 days to about 5.
The third piece is light. Your core body temperature hits its minimum roughly 3 hours before your usual wake time. Light in the 4 hours after that minimum advances your clock (earlier, good for eastbound trips). Light in the 4 hours before it delays your clock (later, good for westbound trips). Light at the wrong time pushes the clock the wrong way, which is why timing matters more than brightness alone.
Think of this page as your personal jet lag planner: it works for any flight route, from a 3-zone hop to a 12-zone haul, and the day-one light therapy schedule it gives you helps you recover faster and beat jet lag rather than just waiting it out.
Enter the number of time zones you will cross (1 to 12) and whether you are flying east or west. Add your usual wake-up time at home as an hour from 0 to 23, since your light windows are anchored to it. If you plan to shift your sleep before departure, enter how many pre-flight shift days you will do (eastbound trips only).
The calculator returns your estimated days to adjust, how many hours your body clock must move and in which direction, your body-temperature minimum, and your day-one seek and avoid bright light windows. Recalculate if your trip details change.
Flying east is harder for most people. Your clock must move earlier, which fights its natural tendency to drift later, and it only manages about 1 hour per day. Flying west asks the clock to move later, which it does more willingly, at about 1.5 hours per day.
That asymmetry shows up in the light advice too. After flying east, seek bright light in the hours after your temperature minimum (morning light at your destination) and avoid it in the evening. After flying west, seek light before your temperature minimum (evening light) and avoid morning light on day one.
Day one sets the tone. For eastbound trips, get outside or use a bright light box during the seek window the calculator gives you, and wear sunglasses or stay indoors during the avoid window. For westbound trips the pattern flips: seek evening light, avoid morning light.
A light box of about 10,000 lux for 20 to 30 minutes works when sunlight is not available. Keep the room dark during the avoid window: blackout shades or an eye mask help more than most people expect.
You can start adjusting up to 3 days before an eastbound flight. Move your bedtime and wake time 1 hour earlier each day, and get bright light right after waking while avoiding bright light in the evening. Each day banks roughly 0.7 zones of credit in the formula.
For westbound trips, pre-shifting is less commonly recommended: move bedtime later by 1 to 2 hours for a couple of nights and seek evening light. Short eastbound shifts of 1 to 2 zones often do not need a pre-flight plan at all.
Sleep on the plane only during what will be nighttime at your destination. On a New York to London evening flight, for example, try to sleep through most of the flight, since you will land in the morning. On a westbound daytime flight, staying awake and seeking light helps you land ready for a late local bedtime.
Avoid long naps on arrival day. If you are exhausted, a 20 to 30 minute nap before mid-afternoon is far less damaging to the adjustment than a 3-hour evening nap.
Maya flies from New York to London: 6 time zones east, usual wake time 7 AM, no pre-flight plan. Days to adjust = ceil(6 / 1) = 6 days. Her temperature minimum is 4 AM, so on day one she seeks bright light from 5 AM to 9 AM and avoids bright light from 5 PM to 9 PM.
If she shifts her sleep 1 hour earlier for 3 days before flying, the bill drops: ceil((6 - 0.7 × 3) / 1) = ceil(3.9) = 4 days... wait, (6 - 2.1) = 3.9, ceil = 4 days. Two extra days of normal energy, earned before takeoff.
Roughly 1 day per time zone when flying east and about two-thirds of a day per zone when flying west. So 6 zones east takes about 6 days and 6 zones west takes about 4 days, assuming well-timed light exposure. Age, sleep debt, and flight timing can stretch this.
East is usually worse. Your body clock must move earlier, against its natural drift, at only about 1 hour per day. Westbound travel asks it to move later, which it manages at about 1.5 hours per day.
Divide the time zones crossed by 1 for eastbound travel or by 1.5 for westbound travel, then round up. Subtract about 0.7 zones per pre-flight shift day if you started adjusting eastbound before departure.
Some travelers use low-dose melatonin to nudge sleep timing, but evidence is mixed and guidance differs by country. Talk to a doctor or pharmacist before using it, especially with other medications or health conditions.
Timing depends on direction and is easy to get wrong, which is why professional guidance matters more than a fixed rule. As a rough pattern people discuss with clinicians: a small dose in the local evening after eastbound travel. Do not guess doses: ask a healthcare professional.
Sleep during what will be nighttime at your destination, and stay awake when it will be daytime there. This single habit does more for most travelers than any supplement.
Shift your sleep 1 hour per day for a few days before an eastbound flight, time bright light correctly on day one, avoid long arrival-day naps, and stay hydrated. You cannot fully avoid it on long eastbound trips, but you can shorten it.
Bright light is the strongest lever you have: it is what moves your body clock. But it must hit at the right time. Light at the wrong time of day pushes the clock the wrong way and can make jet lag worse.
Jet lag typically starts at 2 or more time zones crossed. One zone rarely causes true jet lag, though poor sleep on the flight can still leave you tired.
For trips of 2 to 3 days, many travelers do better staying on home time: keep your home sleep schedule, seek light at home-time mornings, and avoid a full adjustment you will just have to undo.
You can reduce it. Shift sleep earlier before eastbound flights, bank extra sleep the week before, and arrive a day early for important events. Full prevention is not realistic on long eastbound routes.
See a doctor if severe sleep problems last more than a week after travel, or if you fly across many zones frequently for work and never feel adjusted. Frequent flyers can develop a chronic pattern worth treating.
No. This calculator gives planning estimates based on published adjustment rates, not medical advice. For sleep disorders, medication questions, or health conditions, talk to a healthcare professional.
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