Jet Lag Calculator

What is jet lag and why does it happen?

Jet lag is the mismatch between your internal circadian clock and the local time at your destination. Your circadian clock controls body temperature, cortisol production, melatonin release, alertness, digestion, and dozens of other physiological rhythms on a roughly 24-hour cycle. When you cross time zones, the external cues (light, meal times, social activity) shift to a new schedule, but your internal clock continues running on the old one.

The result: you're wide awake at 3am, unable to stay alert at noon meetings, hungry at 2am, and foggy-headed for days. The symptoms aren't just fatigue. Jet lag impairs cognitive performance, reaction time, mood, and gastrointestinal function.

Eastward versus westward: why direction matters

The human circadian clock doesn't run at exactly 24 hours, it free-runs at approximately 24.2 hours. This means it naturally drifts slightly later each day without external resetting cues. Phase delay (moving the clock later, as in westward travel) aligns with this natural drift. Phase advance (moving the clock earlier, as in eastward travel) fights against it.

This is why most people find that flying west (New York → Los Angeles, London → New York) produces milder jet lag than flying the same number of time zones east. The recovery rate for eastward travel is approximately 1 day per time zone; for westward travel, 0.75 days per time zone. A 5-zone westward trip recovers in about 4 days; the same distance east takes 5.

The body clock on arrival

When this calculator shows "your body clock on arrival," it's telling you what time your internal biology thinks it is, regardless of what the local clock says. If you fly from New York to London and arrive at 7am London time, your body clock is registering 2am New York time, deep in the biological sleep window. Your cortisol hasn't surged, your core temperature is at its minimum, and every physiological signal is saying "sleep."

Understanding this explains the first-night strategy: trying to sleep at 11pm London time when your body clock says it's only 6pm New York time is very difficult. A 9pm or 10pm compromise is more achievable and still advances the clock in the right direction.

Light exposure: the most powerful tool

Light resets the circadian clock via the suprachiasmatic nucleus (SCN) in the hypothalamus. Light exposure in the morning advances the clock (makes it earlier); light in the evening delays it (makes it later). For eastward travel, you want to advance, so seek morning light and avoid evening light. For westward travel, you want to delay, seek evening light and avoid early morning light.

The critical practical application: if you're flying eastward and arrive in the morning, get outside immediately. Natural morning light is the most potent zeitgeber available. If you're flying westward, stay up in bright light in the local evening before forcing yourself to sleep.

Melatonin timing

Exogenous melatonin doesn't make you sleep, it signals "darkness" to the circadian clock. Used correctly, it can advance or delay the clock to accelerate adaptation. For eastward travel: take melatonin at your destination bedtime. For westward travel: take melatonin slightly before your desired sleep time at the destination. The dose that matters is low, 0.5–1mg is as effective as higher doses and more physiologically appropriate.

Related tools

Jet lag combines with other sleep disruptions to create larger deficits. Use the Sleep Debt Calculator to assess how much cumulative sleep you lost across the trip. The Sleep Cycle Calculator can help you find optimal wake times at your destination that minimise grogginess during the adaptation period.