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How does jet lag affect the body?

Medicine & HealthLesson on requestFree to read

Quick answer

Jet lag is a temporary misalignment between your body's internal clock and the local time after crossing time zones. The clock, set to your home schedule, controls sleep, hormones and digestion. Until it resets by about an hour a day through light exposure, you feel tired, foggy and off-schedule.

Jet lag happens when your internal body clock, set to your home time zone, is out of sync with the local day-night cycle at your destination. This mismatch disrupts sleep, hormone release, digestion and alertness until the clock gradually resets.

Step by step

  1. Your body's circadian clock, based in the brain's suprachiasmatic nucleus, is set to the light-dark cycle of your home time zone.
  2. A flight across multiple time zones shifts the local day-night schedule abruptly, so the clock no longer matches the environment.
  3. The clock continues to send sleep and wake signals on home time, causing alertness at night and sleepiness during the day.
  4. Organs such as the liver and gut have their own clocks that also fall out of step, leading to digestive problems.
  5. Light exposure at the new destination gradually resets the master clock, and it shifts by about one hour per day.
  6. Once the clock realigns with local time, sleep, hormone rhythms and digestion return to normal.

Key things to understand

  1. Your body runs on a roughly 24-hour internal clock called the circadian rhythm, which controls sleep, alertness, hormones and digestion.
  2. That clock is normally reset each day by light hitting the retina, which signals the brain's master pacemaker, the suprachiasmatic nucleus.
  3. Flying across time zones changes the timing of light and darkness, so the clock's signals no longer match the local environment.
  4. The clock can shift only about one hour per day, so crossing six time zones may take nearly a week to adjust fully.
  5. While the clock is out of step, you may feel sleepy at the wrong times, wake at night, and have poor appetite or digestion.

Labelled diagram

  1. Normal rhythm

    The circadian clock aligns with local light and dark, controlling sleep and hormones.

  2. Time zone shift

    A flight changes the local day-night schedule, but the internal clock stays on home time.

  3. Misalignment

    The clock sends sleep signals at the wrong local time, causing fatigue and poor focus.

  4. Organ clocks

    Digestive and other organ clocks fall out of step, leading to appetite and stomach issues.

  5. Gradual reset

    Light exposure at the new destination slowly shifts the clock by about an hour per day.

How jet lag disrupts the body clock

Real-world examples

  • A traveler flying from London to Mumbai crosses about five time zones and may feel sleepy in the afternoon but wide awake at 3 a.m.
  • A businessperson flying from New York to Tokyo for a two-day meeting often finds it hard to concentrate because their body is still on New York time.
  • Flight crews on long-haul routes between the US and Australia frequently experience jet lag because they cross many time zones repeatedly.
  • A family returning from a holiday in Thailand to the UK may wake up hungry in the middle of the night for several days.

Common mistake

✗ Myth: Jet lag is just a fancy name for being tired after a long flight.

✓ Fact: Jet lag is a specific disruption of the body's circadian rhythms caused by crossing time zones, not simply fatigue from travel. It affects sleep, hormone release and digestion, and it takes days to resolve as the internal clock resets.

People also ask

What causes jet lag?
Jet lag is caused by a mismatch between your internal body clock, which is set to your home time zone, and the local day-night cycle at your destination. When you cross time zones quickly, your clock is out of sync with the environment, disrupting sleep, alertness, hormone release and digestion until it gradually resets.
How to get over jet lag faster?
Getting over jet lag faster usually involves exposing yourself to daylight at the new destination at appropriate times, which helps reset the body clock. Keeping a regular sleep schedule, avoiding heavy meals at odd hours, and staying hydrated may also help. The clock shifts only about an hour a day, so full adjustment takes time.
Why does jet lag make you feel sick?
Jet lag can make you feel sick because your digestive system has its own circadian clocks that expect meals at certain times. When you eat at times that conflict with your internal clock, you may experience nausea, indigestion, constipation or diarrhea until the clocks realign with the new local schedule.
Why is jet lag worse when flying east?
Flying east usually causes worse jet lag because you must go to bed earlier and wake earlier, but the body clock naturally runs slightly longer than 24 hours, making it easier to delay sleep than to advance it. Westward travel aligns better with that natural tendency.
How long does jet lag usually last?
Jet lag typically lasts about one day for each time zone crossed, though it varies. Most people need a few days to a week to fully adjust, depending on direction of travel, age and individual differences.
What happens in the body during jet lag?
The master clock in the brain and clocks in organs like the liver and gut fall out of sync with each other and with the outside world. This disrupts sleep, body temperature, hormone release such as cortisol and melatonin, and digestion, causing fatigue, poor concentration and stomach upset.
Can jet lag affect digestion and appetite?
Yes. The digestive system has its own circadian clocks that expect meals at certain times. When you eat at times that clash with your internal clock, you may experience indigestion, constipation, diarrhea or loss of appetite until the clocks realign.

Part of the guide: How Your Body Works →