Jet lag is the mismatch between your internal 24-hour body clock and the local time at your destination. Flying across several time zones in a few hours forces your circadian rhythm to advance or delay overnight, and the result is daytime sleepiness, broken nights, sluggish focus, and a stomach that refuses to eat at local mealtimes. This jet lag calculator — a free time zone change recovery calculator — estimates how many days your body needs to resync after a long flight, compares eastbound versus westbound travel, and gives you a timed light-exposure plan so you can arrive ready instead of wiped out.
The single most important fact about jet lag is direction. Going east — toward the rising sun — your body clock has to advance, which most people find harder and slower. Going west, your clock has to delay, which is usually easier because your internal day is naturally a little longer than 24 hours. That is why a New York-to-Tokyo trip and the reverse trip do not cost the same number of recovery days, even though both cross 14 time zones.
How to Use
- Enter the Time Zone Difference in hours. Use a positive number when you fly east and the destination clock is ahead of home (for example, New York to Tokyo is +14). Use a negative number when you fly west and the destination is behind (Tokyo back to New York is −14).
- Type your Age. Recovery slows with age: travelers over 60 typically need about 20% more time, while children and teenagers often adapt a little faster.
- Set Daily Bright Light to the outdoor daylight you realistically plan to get at your destination. Thirty minutes or more of morning sunlight shortens recovery by roughly 15%; fifteen minutes gives a smaller boost; none means you rely on sleep alone.
- Read the result card. Baseline Recovery is the rule-of-thumb days for the direction alone; Estimated Recovery factors in your age and light exposure; Severity bands the trip as Mild, Moderate, or Severe.
- Use the light plan below the numbers. Eastbound travelers should seek morning light and avoid bright light late in the evening; westbound travelers should seek evening light and protect morning sleep with sunglasses.
The Recovery Rule of Thumb
Sleep researchers and travel-medicine clinics converge on a simple estimate: recovery takes roughly one day per time zone crossed eastbound, and a conservative rule adds about 1.2 days per zone westbound to account for the slower, gradual delay. Expressed as recovery days per time zone crossed:
- Eastbound (phase advance): about 1.0 day per zone. A 14-hour eastward jump is therefore about 14 days of baseline recovery.
- Westbound (phase delay): about 1.2 days per zone. The same 14 hours going west is about 16.8 days on this conservative rule.
From that baseline the calculator applies two multipliers. Light exposure ramps linearly: 30+ minutes cuts recovery by 15% (factor 0.85), 15 minutes cuts it by about 7.5% (factor 0.925), and 20 minutes cuts it by about 10% (factor 0.90). Age adds 20% for travelers 60 and over (factor 1.2) and trims 10% for travelers 18 and under (factor 0.9). Worked on the New York-to-Tokyo example: 14 zones eastbound gives a 14.0-day baseline; with 30 minutes of daily light at age 30 that becomes 14.0 × 0.85 = 11.9 days.
Why Eastbound Feels Worse
Your circadian pacemaker runs on a cycle slightly longer than 24 hours on average. Delaying it (staying up later) is therefore more natural than advancing it (forcing yourself to sleep earlier). Eastbound travel demands an advance — your body must wake up and go to sleep earlier than it wants — and most people stall for a few days before the shift sticks. Westbound travel asks for a delay, which aligns with that slightly-longer-than-24-hour tendency, so most travelers feel functional sooner.
This asymmetry is the reason frequent flyers deliberately adjust their schedule before an eastbound trip: waking up 30 to 45 minutes earlier for a couple of days, and getting bright light immediately upon waking, pre-loads the advance. For westbound trips the opposite works — shift bedtime later and seek evening light.
Examples
Example 1 — New York to Tokyo (eastbound, 14 hours). A 30-year-old crosses 14 time zones east with 30 minutes of daily light. Baseline recovery is 14.0 days; the light adjustment brings the estimated recovery to 11.9 days, rated Severe. Morning light on arrival and avoided evening screens are the priorities.
Example 2 — Tokyo to New York (westbound, 14 hours). The same traveler going west crosses the same distance; the direction factor here is 1.2, so baseline recovery is 16.8 days, and without deliberate light exposure the estimated recovery stays at 16.8 days (rated Severe). Evening light and a dark, cool bedroom help lock in the delay.
Example 3 — A short business hop. Flying from Los Angeles to New York is a 3-hour eastbound jump at age 45 with 20 minutes of light. Baseline recovery is 3.0 days; the linear light ramp trims it to about 2.7 days, and severity is Mild — one good night usually does the job.
Example 4 — A senior traveler to London. A 65-year-old flies 5 hours east and gets 15 minutes of light. Baseline is 5.0 days; age adds 20% (factor 1.2) and the light ramp trims about 7.5% (factor 0.925), landing near 5.6 days — plan a buffer day before the first important meeting.
Light Exposure Schedule
- Eastbound trips: get bright outdoor light in the first half of the day at your destination; dim lights and avoid screens after 10 PM; if needed, use a low-dose melatonin about an hour before target bedtime.
- Westbound trips: seek bright light in the late afternoon and early evening; keep morning light low with sunglasses until your target wake time; avoid naps that push bedtime later.
- Across 8+ zones: consider splitting the trip with an overnight stop so the clock shifts in two smaller steps rather than one jump.
FAQ
How long does jet lag last?
Most people recover from jet lag at roughly one day per time zone crossed eastbound, and this calculator uses a conservative 1.2 days per zone westbound. A 14-hour eastward trip typically takes 11 to 14 days to fully resync; the same trip westbound budgets about 17 days. Severity, age, and light exposure all change the exact number.
Is eastbound or westbound jet lag worse?
Most people find advancing the body clock — forcing an earlier bedtime and wake time — harder than delaying it, because the natural circadian cycle runs slightly longer than 24 hours. This calculator takes a conservative stance on westbound recovery too, budgeting more days per zone, so you always plan a buffer rather than assuming you will bounce back quickly.
Does bright light really speed up recovery?
Yes. Morning outdoor light is the strongest signal your circadian pacemaker receives. Thirty minutes or more of daylight at the destination can cut recovery by about 15%, because it directly tells the internal clock what time it is. Indoor light is far weaker; sunglasses and staying in a dim hotel room blunt the effect.
How does age affect jet lag?
Recovery tends to slow with age. Travelers over 60 commonly need about 20% more days than the baseline estimate, while teenagers often adapt a little faster. Older flyers also lose some circadian amplitude, so keeping a strict sleep schedule and protecting daytime light matters even more.
Should I take melatonin for jet lag?
Melatonin can help anchor an earlier bedtime on eastbound trips when taken about an hour before the target local sleep time, but it is a supplement, not a fix, and dosing varies. This calculator does not provide dosing advice; talk to a doctor or pharmacist, especially if you take other medications.
When should I adjust my schedule before the flight?
For eastbound trips, start waking up 30 to 45 minutes earlier and getting morning light two to three days before departure. For westbound trips, shift bedtime later and seek evening light. Small pre-trip shifts reduce the size of the jump the body has to make in one night.
What is the fastest way to recover after landing?
Set your watch to destination time the moment you board, get sunlight at the correct local time, avoid long daytime naps, eat meals at local mealtimes, and keep the first night's sleep environment dark, cool, and quiet. Combining timed light with a fixed sleep schedule beats caffeine-only strategies.
Related Calculators
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Disclaimer
This jet lag calculator is provided for informational purposes only and is not medical advice. Estimates are based on the widely used per-time-zone rule of thumb plus light and age multipliers; individual recovery varies widely. This tool does not diagnose, treat, cure, or prevent any medical condition, and it is not a substitute for advice from a qualified physician, travel-medicine clinic, or sleep specialist. If you have a sleep disorder, cardiac condition, or are pregnant, consult your doctor before changing sleep, light, or supplement routines around travel. For our editorial standards see the About page, and for data handling see the Privacy Policy; we do not store the travel details you enter on this page.