The usual estimate is about one day of adjustment for each time zone you cross, so a six hour shift often means most of a week. Jet lag is not a shelf life or a spoilage clock. It is a body clock still running on the city you left, and it moves slowly.
Your body clock is still on the old city
Humans run on an internal daily rhythm coordinated by a small cluster of cells in the brain called the suprachiasmatic nucleus. It governs when you feel alert, when body temperature dips, when digestion speeds up, and when sleep pressure builds. Left in a cave with no cues, that rhythm keeps running on a cycle very slightly longer than twenty four hours.
Light is the main signal that keeps it lined up with the real world. Fly across six zones and the light arriving at your eyes now contradicts everything the internal clock expects, so the two have to renegotiate. That negotiation is gradual, roughly an hour a day for most people, which is where the one day per zone estimate comes from.
Eastward and westward do not cost the same
| Zones crossed | Flying west, rough adjustment | Flying east, rough adjustment |
|---|---|---|
| 1 to 2 | A day or less | One to two days |
| 3 | Two days | Three days or so |
| 5 to 6 | Three to four days | Five to six days |
| 8 to 9 | Five days or so | A week or more |
| 11 or more | Under a week, often | Over a week, commonly |
The asymmetry has a tidy explanation. Flying west stretches your day, and a clock that naturally wants to run slightly long finds that easy to absorb. Flying east compresses the day and asks the clock to go to bed before it is ready, which it resists. Anyone who has flown London to New York and then back again has felt the difference.
Signs you have actually adjusted
The clearest marker is waking up without an alarm at a time that makes sense locally, two or three mornings in a row. Before that point people often fall asleep easily and then snap awake at three in the morning, which is the old schedule asserting itself.
Appetite is the second signal, and it lags behind sleep. Feeling genuinely hungry at local lunchtime rather than at eleven at night means digestion has caught up. The third is the afternoon slump moving to where locals have theirs instead of flattening you at ten in the morning.
Daylight is the lever most travelers underuse. Getting outside during the local morning after flying east, and during the local afternoon after flying west, is the standard advice for nudging the clock in the direction you want.
When it is worth talking to a doctor
This article covers the general pattern, not your situation. Anything involving sleep medication, supplements, or timed light therapy is a conversation for a clinician who knows your history, and that includes melatonin, which is sold over the counter in the United States but is not a casual decision for everyone.
The moments people commonly raise with a doctor are symptoms that outlast the trip by a wide margin, travel that repeatedly disrupts an existing sleep problem, and any condition on a fixed daily medication schedule, since crossing zones changes what the schedule means. Frequent crew and shift workers often have occupational guidance available to them as well.
Frequently asked questions
Does jet lag happen on short flights?
Only if you change time zones. A long north to south flight, say New York to Bogota, crosses almost no zones and leaves the body clock where it was. Travel fatigue from a cramped cabin and a short night is a separate thing, and it usually clears after one good sleep.
Is jet lag worse as you get older?
Many travelers report that it is, and the general explanation offered is that the body clock becomes less flexible and sleep becomes lighter over the years. Individual experience varies a great deal, so treat that as a broad pattern rather than something that will apply to you.
Why do some people feel fine after a long flight?
Body clocks differ in how strongly they resist being shifted, and habits matter too. Someone who already keeps an irregular schedule, gets a lot of outdoor daylight, and sleeps well on planes has a much easier time than someone locked into a rigid routine at home.
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