How does the jet lag planner work?
- Pick your cities. Choose where you are flying from and to. If your city is not listed, pick one in the same time zone.
- Set your travel date. The planner reads both time zones on that date, so daylight saving time at either end is counted.
- Enter your usual sleep. Your normal bedtime and wake time at home. The plan keeps the same times on local time once you land.
- Choose how many days you can prepare. Each day before the flight moves your sleep one hour toward your destination. None is fine too.
- Follow your plan. Each day shows when to sleep, when to get bright light, when to avoid it, and when to stop caffeine. You can print it or save it as a PDF.
The example already filled in is a flight from New York to London, five hours ahead, for someone who sleeps from 11pm to 7am. Nothing you enter is stored.
What causes jet lag?
Your body runs on an internal clock that sets when you feel sleepy, alert and hungry. After a long flight across time zones, that clock is still set to home, while the sun and the clocks around you have jumped ahead or behind. The mismatch is jet lag.
The CDC's Yellow Book, its guide for travel health, describes jet lag disorder as sleep problems that follow travel "across at least two time zones", with symptoms that appear "within 1–2 days after travel." Typical complaints are poor sleep at night, sleepiness during the day and feeling generally unwell. For how the body clock works in everyday life, read what your circadian rhythm is and why it controls so much.
How long does jet lag last?
It depends on how many time zones you cross and which way you fly. The CDC gives "an average rate of adaptation of 1.5 hrs/day for westward travel and 1 hr/day for eastward travel." The planner uses those two averages.
| Time difference | Flying east | Flying west |
|---|---|---|
| 3 hours | About 3 days | About 2 days |
| 6 hours | About 6 days | About 4 days |
| 9 hours | About 9 days | About 6 days |
These are averages. Some people adjust faster and some slower, and the light you get on each day makes a real difference, which is what the plan is built around.
Why is flying east harder than flying west?
Flying east asks your body clock to move earlier, and flying west asks it to move later. The human clock runs slightly longer than 24 hours, so, as a 2019 review in Frontiers in Physiology puts it, "it has a greater propensity to delay than to advance." Staying up later comes more easily than going to sleep earlier.
For very large changes this has a surprising result. When a destination is 10 or more hours ahead, moving your clock later the long way round, for example 14 hours later instead of 10 hours earlier, is quicker at those average rates. The planner checks both routes and shows the quicker one.
How does light shift your body clock?
Daylight is the strongest signal your body clock uses, and its effect depends on timing. The clock has a daily low point, roughly when your body temperature is lowest. The same review gives the example of someone who sleeps from 11pm to 7am, whose low point falls at about 4am, around three hours before waking.
- Light before the low point pushes the clock later. The review says light "in the ~12 h prior to CBTmin", the low point, causes a delay.
- Light after the low point pulls the clock earlier. Light "in the ~12 h after CBTmin" causes an advance.
After a long flight east, your low point lands in the local morning. Bright morning light at your destination then falls before it and pushes your clock the wrong way. For an 8-hour flight east, the review suggests it is better "to avoid sunlight before midday" and get daylight in the afternoon. That is why the plan sometimes tells you to avoid bright light in the morning and seek it later, and why the windows move each day as your clock catches up.
The planner places your low point three hours before your usual wake time and concentrates on the three hours either side of it, where light has the most effect. It is an estimate: everyone's clock is a little different.
Can you prepare for jet lag before you fly?
Yes. The CDC suggests "shifting sleep 1 hour later (for westward travel) or earlier (for eastward travel) per day in the 2–3 days prior to the trip." Every hour you shift at home is an hour less to absorb when you land.
The trade-off is real. The 2019 review notes that shifting before travel "could interfere with sleep and be socially disruptive prior to travel", for example with early mornings before work or late nights around family. Choose the number of days that fits your life, including none.
What about caffeine, naps and melatonin?
- Caffeine. The CDC suggests using caffeine as needed for alertness during the day at your destination and to "Stop 6 hours before bedtime." The plan shows that cut-off each day. Our caffeine calculator shows how much is still in your system at bedtime.
- Naps. "Short daytime naps (20–30 minutes) can be utilized to help sustain alertness during the local day", while longer naps may make the night's sleep harder.
- Melatonin and sleep medicine. The CDC explains that melatonin's effect depends on when it is taken, and that "taking it at the wrong time can increase misalignment." It also notes that melatonin is not regulated by the FDA, so the amount in a product may differ from the label. Talk to a doctor or pharmacist before using melatonin or sleep medicine for travel.
What does the planner not account for?
- Your own body clock. It uses average rates and a typical low point. Natural early risers and night owls may adjust at different speeds.
- Flight times. It does not know when you take off or land, or whether you sleep on the plane. On the day you arrive, start with whatever part of the plan still lies ahead.
- Weather and daylight hours. If the windows fall when it is dark, bright indoor light helps less than daylight but is better than none.
- Health conditions. If you have a sleep disorder, take regular medication at set times, or have a condition that needs careful timing, plan your trip with your doctor.
For everyday sleep, the sleep hygiene checklist and the sleep calculator cover the basics, and the social jetlag calculator shows the smaller shift many people put their clock through every weekend. The full guide to sleep covers the rest.
