Jet Lag Calculator

Jet lag is the mismatch between the clock on the wall at your destination and the internal body clock that is still keeping home time. The usual signs are sleepiness in the afternoon, wakefulness in the middle of the night, a dulled appetite or an unsettled stomach, and trouble concentrating. This calculator asks for just two things, the number of hours by which your body clock has to shift and the direction you flew, and turns them into a rough number of days. Use it to decide how much slack to leave around a presentation, a long drive or an important appointment after a flight. It is general information, not medical advice, and it cannot diagnose anything.

Why flying east usually feels worse than flying west

The body clock is commonly described as running slightly longer than 24 hours when it gets no outside cues. If that is true, stretching your day is easier than squeezing it. Flying west lengthens your day: you go to bed later than usual, which the clock tolerates fairly well. Flying east shortens your day: you must fall asleep and wake up earlier than your body expects, which is harder. That is the whole reason this tool uses a bigger multiplier for eastbound trips. It is a general pattern rather than a law, and some people feel it strongly while others hardly notice it.

The arithmetic

The tool uses a simple rule of thumb: eastward = hours of shift × 1.0 day and westward = hours of shift × 0.67 day, rounded to one decimal place. In other words, roughly one day of adjustment per hour when you go east and about two thirds of a day per hour when you go west. The 0.67 comes from a common description in which the body clock catches up by about one hour a day eastward and about an hour and a half a day westward, and 1 divided by 1.5 is about 0.67. Sources quote different numbers, so treat both factors as a convenient approximation. They are not measured constants, and they do not capture how deeply an individual's schedule is anchored.

Reference table 1: estimated days by hours of shift

The gap between the two directions grows with the size of the shift. At 12 hours the rule gives 12 days eastward against 8 westward, a four-day difference. At 1 or 2 hours the numbers are so small that the direction hardly matters.

Reference table 2: examples departing from New York

The clock difference is the gap between New York standard time (UTC-5) and the destination's standard time. Places that observe daylight saving time are one hour different during that period, so check the real difference before you fly; the World Time Zone Converter will do that. When the clock difference is more than 12 hours, the calculator assumes your body takes the shorter way round and uses 24 minus the difference. A trip with a 14-hour clock gap counts as a 10-hour shift. Around 12 hours, people differ in whether they adjust by moving earlier or later, and recovery can look different from the estimate.

Worked examples

• New York to Paris (eastward, 6 hours): 6 × 1.0 = 6.0 days. A trip that is shorter than a week may be over before the body has fully caught up, so plan the early days gently.
• London to New York (westward, 5 hours): 5 × 0.67 = 3.35, shown as 3.4 days. Compare it with the opposite direction: the same five hours eastward would be 5.0 days.
• New York to Los Angeles (westward, 3 hours): 3 × 0.67 = 2.01, shown as 2.0 days. At around three hours many people feel little more than an early evening yawn, which is why the tool labels anything under 3 hours as mild.

Common misunderstandings and limits

• "One good night's sleep fixes it." Falling asleep because you are exhausted is not the same as having your body clock match local time. Early waking and a second slump in the afternoon can last several days.
• The number of days is not a countdown to 100%. It is better read as a window in which you may feel below your best, with the worst of it usually at the start.
• Short trips can be handled differently. If you will be away for two or three days, some people keep their home sleep schedule instead of adjusting. This calculator is aimed at longer stays and at recovering after you come home.
• What it ignores. Age, usual sleep habits, sleep during the flight, arrival time, cabin conditions, medication, shift work and health conditions are not part of the calculation. Please talk to a doctor or pharmacist before using sleep aids or supplements, and do not drive if you are drowsy.
• It is not a diagnosis. Persistent insomnia, severe daytime sleepiness or other symptoms are a reason to see a professional rather than rely on an estimate.

Ways people try to adjust faster

• Before an eastbound flight, some travelers move bedtime and wake time about an hour earlier each day for two or three days, and do the opposite before a westbound flight.
• Set your watch to destination time when you board and try to sleep when it is night there and stay awake when it is day.
• After arrival, spend time outdoors in daylight. For eastward trips, morning light is usually described as more helpful, and for westward trips afternoon light.
• Go easy on caffeine and alcohol on the plane and in the first days, because both can disturb sleep. If you nap, keep it short, around 20 to 30 minutes.

These are common suggestions, not guarantees, and they will not suit everyone.

Turning the estimate into a plan

Read the figure as a period in which to avoid scheduling anything that needs sharp judgment. If the result is about five days, arrive two days before an important meeting rather than the night before, and after you return, keep the first days of work light. To fine-tune bedtimes, try the Sleep Cycle Calculator, which counts back in 90-minute cycles. If you are wondering when your last cup of coffee will have worn off, see the Caffeine Half-Life Calculator. You can also compare local times in theTime Zone Converter and prepare with the Travel Packing Checklist.

Frequently Asked Questions