JET LAG
The circadian rhythm explained for long-haul travellers
Your circadian rhythm is the roughly 24-hour internal cycle that controls when you sleep, wake, digest and release hormones. It is not one clock but many: a master clock in the brain sets the time, and separate clocks in your organs keep their own. After a long-haul flight these clocks fall out of step with each other and with local time, and that internal disagreement is what jet lag really is. This guide explains how the system works, and why full recovery takes longer than the few days most advice promises.
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What is the circadian rhythm?
In brief
The circadian rhythm is a self-sustaining cycle of close to 24 hours governing sleep, body temperature, digestion and hormones. It runs even in constant darkness, so it needs daily cues to stay aligned with the outside world.
The circadian rhythm is a self-sustaining biological cycle of close to 24 hours that governs sleep and wakefulness, body temperature, digestion, and the release of hormones such as melatonin and cortisol. The word comes from the Latin circa diem, meaning “about a day.” It runs on its own even in constant darkness, which is why it is called endogenous: it is generated inside the body, not imposed by the sun. Left to itself the human clock runs slightly longer than 24 hours, so it needs daily cues to stay aligned with the outside world.
How does the body clock keep time?
In brief
Timekeeping starts in the suprachiasmatic nucleus (SCN), around 20,000 neurons in the hypothalamus that read light directly from the eyes. Inside each clock cell, a 24-hour genetic feedback loop sets the timing of melatonin and cortisol.
Timekeeping starts in the suprachiasmatic nucleus (SCN), a cluster of around 20,000 neurons in the hypothalamus. The SCN receives light information directly from the eyes through the retinohypothalamic tract, which is why light is the body’s strongest timing cue.
Inside each clock cell, time is kept by a genetic loop. Two proteins, CLOCK and BMAL1, switch on the genes PER and CRY; the PER and CRY proteins then build up and switch their own genes back off. One full turn of this transcription–translation feedback loop takes about 24 hours (Takahashi, Nature Reviews Genetics 2017). The SCN uses this loop to set the timing of melatonin release from the pineal gland at night and the cortisol peak in the morning. When those outputs drift out of sync with local time, you get the disorientation described in our overview of what jet lag is.
You do not have one clock, you have many
In brief
The SCN is the master clock, but almost every organ carries its own peripheral clock: liver, gut, kidneys, muscle, even fat cells. These peripheral clocks do most of the practical work and answer to different cues, so they reset at different speeds.
The SCN is the master clock, but it is not the only one. Almost every organ carries its own peripheral clock running the same genetic loop: the liver, the gut, the kidneys, the muscles and even individual fat cells. In normal life the SCN keeps these peripheral clocks loosely coordinated, mostly through body temperature, hormone rhythms and your daily pattern of eating and moving.
These peripheral clocks do most of the practical work. The liver clock times the enzymes that handle a meal; the gut clock times digestion and motility; the muscle clock influences when you are physically at your best. Because they answer to different cues, they can be reset at different speeds, which becomes the central problem after travel. For the trigger side of this, see what causes jet lag.
Why the clocks fall out of step after a flight
In brief
After crossing time zones the master clock shifts within days, driven by light, but the organ clocks lag because their cues (meals, activity) lag too. That internal disagreement, not just tiredness, is the real engine of jet lag.
Cross several time zones quickly and the SCN, driven by the new light–dark cycle, begins to shift within a few days. The peripheral clocks shift more slowly, because the cues that reset them, meal timing, activity, temperature, lag behind. For a while the master clock is on destination time while the liver and gut are still closer to home. This state is called internal desynchronisation, and it is the real engine of jet lag: not simply being tired, but having organs that disagree about what time it is.
Research in both animals and people shows peripheral clocks can take one to two weeks to realign fully after a large shift, well beyond the point at which sleep feels normal. The figure below gives a rough picture for a large eastward shift, where the body has to advance its clock, the harder direction, as explained in why eastward travel is worse.
How long each clock takes to realign
Indicative realignment after a large eastward shift. The sleep–wake clock resets first; the metabolic clocks set the true pace of recovery.
This is why most consumer advice, which stops at “your body clock takes a few days to adjust,” is only half right. The sleep–wake signal does adjust in a few days. The deeper metabolic clocks take roughly a week to two, which is why appetite, digestion and energy can stay off for longer than your sleep does. A fuller breakdown sits in our guide to how long jet lag lasts.
Cortisol, and the wired-but-tired feeling
In brief
Cortisol normally surges in the hour after waking, then tapers. After a large time-zone shift that surge fires at the wrong local time, and the cortisol rhythm can take around 11 days to re-adapt, producing the wired-but-tired state.
Cortisol is the clearest example of a rhythm that travel knocks out of place. Normally it surges in the hour after waking, the cortisol awakening response, then tapers through the day. This is what lifts you into the morning. After crossing six or more time zones, that surge fires at the wrong local time, and full re-adaptation of the cortisol rhythm has been measured at around 11 days (Lac & Chamoux, salivary cortisol in eastward travellers).
That mistiming explains the familiar “wired but tired” state: a cortisol pulse arriving in your local evening leaves you alert when you want to sleep, while its absence in the local morning leaves you flat when you need to perform. Because cortisol also helps reset the peripheral clocks, its disruption slows the whole system. If you wear an Oura, Whoop or Garmin, this is part of why heart rate variability and resting heart rate can stay disturbed for days after you subjectively feel recovered.
What resets each clock
In brief
Each clock answers to a different cue, or zeitgeber. Light resets the master clock; meal timing is the dominant cue for the liver and gut. Recovery uses both: timed daylight plus eating on destination meal times.
Each clock answers to a different cue, or zeitgeber, German for “time-giver.” Knowing which cue resets which clock is what turns the biology into something you can act on.
Light is the master cue and resets the SCN. Correctly timed daylight, and avoidance of light at the wrong time, is the single most powerful lever for the sleep–wake clock. But light has little direct effect on the liver. In a now-classic experiment, restricting feeding to the wrong time of day shifted the liver clock by about 10 hours within two days while the SCN stayed locked to the light cycle (Stokkan et al., Science 2001). Meal timing, in other words, is the dominant cue for the digestive clocks.
| Clock or system | What it governs | Main resetting cue | Rough realignment time |
|---|---|---|---|
| Master clock (SCN) | Overall timing; the sleep–wake signal | Light | A few days |
| Cortisol rhythm | Morning alertness and stress response | SCN and light | Around 11 days |
| Liver clock | Metabolism and the timing of digestion | Meal timing | Up to about two weeks |
| Gut and muscle clocks | Digestion, appetite, physical readiness | Meal timing and activity | One to two weeks |
The practical reading for a traveller: get outdoor light at the right end of the destination day to move the master clock, and eat on destination meal times, even when you are not especially hungry, to pull the liver and gut clocks along with it. Activity and a cooler night-time temperature reinforce both. Aligning these cues, rather than relying on light alone, is what shortens the window of internal desynchronisation.
What this means for recovery
In brief
Resetting every clock takes time, and the metabolic clocks set the pace, not your sense of tiredness. Timing light and meals to the destination shortens the window; stimulants tend to lengthen it by masking the reset signals.
The honest summary is that resetting every clock takes time, and the metabolic clocks set the pace, not your sense of tiredness. You can feel fine on day three and still be running an internal disagreement that affects digestion, mood and physical output. Timing your light and meals to the destination shortens that window; pushing through on stimulants tends to lengthen it by masking the signals the body uses to reset.
There is also a physiological layer beneath the clock: how efficiently the body restores autonomic balance and circulation after a flight. Some long-haul travellers want to support that recovery rather than wait it out. Aurion Reset is a clinician-supervised recovery protocol at the Aurion clinic in Mayfair, designed for travellers landing into a demanding schedule. A session lasts 75 minutes, including a clinician consultation and 45 minutes of active PureFlow™, heart-synchronised pneumatic compression designed to support circulation and oxygenation while you remain at rest.
Frequently asked questions
How long does it take for your circadian rhythm to reset after a flight?
The master clock that drives your sleep–wake cycle usually shifts within a few days, at a rate of roughly one hour per day. The deeper clocks in your liver, gut and muscles take longer, commonly one to two weeks after a large time-zone change. This is why your sleep can feel normal while your appetite, digestion and energy stay off for a while afterwards.
What is the difference between the master clock and peripheral clocks?
The master clock is the suprachiasmatic nucleus in the brain, which reads light and sets overall timing. Peripheral clocks are the separate clocks in organs such as the liver, gut and muscles, which run the same genetic loop but respond mainly to meal timing and activity. In normal life the master clock keeps them coordinated; after travel they fall out of step, which is the core of jet lag.
Can you reset your body clock with food?
You can reset part of it. Meal timing is the dominant cue for the clocks in your liver and gut, so eating on your destination’s schedule helps pull those clocks into line. Food has little effect on the master clock in the brain, which is governed by light. The most effective approach combines correctly timed daylight for the master clock with destination meal times for the digestive clocks.
Why do I feel hungry at strange times after a long flight?
Because your gut and liver clocks are still partly on home time. Appetite, digestion and the enzymes that process food are timed by these peripheral clocks, and they realign more slowly than your sleep does. Eating at local meal times, even without much appetite, helps reset them and usually settles the odd hunger pattern within several days.
What is the strongest cue for resetting the circadian rhythm?
Light is the strongest cue for the master clock in the brain, delivered through the eyes to the suprachiasmatic nucleus. For the peripheral clocks in the digestive organs, meal timing is the dominant cue. Effective recovery uses both: timed daylight to move the master clock, and destination meal times to move the metabolic clocks with it.
When the next day matters
Aurion Reset is a clinician-supervised recovery protocol at our private clinic in Mayfair, designed for travellers who land tired and need to be at full capacity the morning after.
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