Roughly a day per time zone crossed. Getting the light timing right speeds it up; getting it wrong makes it worse.
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Jet lag is what happens when your body clock is still on one continent and you are on another. Cross several time zones and your internal timing for sleep, alertness, appetite, digestion and hormone release stays where it was, and it takes days to catch up.
The rough rule is one day of adjustment per time zone crossed, though it varies a good deal between people.
Flying east is harder than flying west, and this is not just folklore. Going west means staying up later, which the body clock does willingly — its natural period runs slightly longer than 24 hours. Going east means going to bed earlier than your body wants to, which it resists.
The strongest lever is light, and the timing matters more than the amount. Light at the right hour shifts your clock in the direction you need; the same light a few hours out shifts it the wrong way and makes things worse. Most people get this backwards, which is why some travellers seem to stay wrecked for a week.
Symptoms usually begin a day or two after arrival and ease over several days. Longer for eastward travel and for more time zones crossed.
Travel fatigue is exhaustion from the journey itself — cramped seats, dry cabin air, disrupted sleep, dehydration. It happens on long north–south flights too, where no time zones are crossed, and it clears within a day of proper rest.
Jet lag is the clock being in the wrong place, and rest alone does not fix it.
Fever, diarrhoea, or feeling genuinely unwell after travel is not jet lag. If you have been anywhere with malaria, a fever needs same-day assessment.
Dry cabin air, low pressure and hours of sitting make you feel rough, and worsen how jet lag feels — but they are travel fatigue, not jet lag. Drinking water helps the former and does nothing for the latter.
Jet lag is a clinical diagnosis: symptoms that follow rapid travel across at least two time zones, in a pattern that fits the direction of travel. No test confirms it, and none is needed.
Symptoms lasting more than a couple of weeks after returning are not jet lag, and are worth investigating properly.
Jet lag is well suited to a consultation before you fly, because almost everything useful is a plan rather than a prescription.
We do not prescribe sleeping tablets — no benzodiazepines and no z-drugs, for jet lag or anything else, at any time. That is a fixed position, not a case-by-case one. They fragment sleep quality, cause next-day impairment, and carry real risk on long flights.
Your body clock is most sensitive around the time of your lowest core temperature, roughly two hours before you normally wake. Light after that point pulls your clock earlier; light before it pushes your clock later. The practical version:
The evidence is best for eastward travel across five or more time zones, taken at bedtime at the destination. In the UK it is a prescription-only medicine, and its use for jet lag is outside the licensed indication — so it needs a proper conversation rather than a purchase.
Timing matters more than dose, and taking it at the wrong hour can shift your clock the wrong way.
Move around every couple of hours, keep hydrated, and consider compression stockings if you have risk factors. Jet lag will not harm you; a deep vein thrombosis will.
If you want the honest answer: for a straightforward holiday, the light-timing advice on this page is most of what you need and costs nothing. Book a consultation when medication or an existing condition makes it more than a scheduling problem.
This page follows NICE Clinical Knowledge Summaries on insomnia and jet lag, American Academy of Sleep Medicine practice parameters for circadian rhythm sleep–wake disorders, and NaTHNaC / TravelHealthPro guidance on flying.
Reviewed against NICE CKS, AASM practice parameters and NaTHNaC guidance current at the date shown above.
Because your body clock naturally runs slightly longer than 24 hours, so staying up later comes easily and going to bed earlier does not.
Flying west asks your clock to delay, which it does willingly. Flying east asks it to advance, which it resists.
Roughly a day per time zone crossed, and longer going east.
Getting the light timing right speeds this up considerably; getting it wrong can drag it out for a week.
Flying east: morning light, avoid late evening light. Flying west: evening light, avoid early morning light.
Outdoor light is far stronger than any indoor lighting, so go outside rather than sitting under a lamp.
It has the best evidence for eastward travel across five or more time zones, taken at bedtime at the destination.
It is prescription-only in the UK and its use for jet lag is off-label, so it needs a conversation — and the timing matters more than the dose.
No. We do not prescribe benzodiazepines or z-drugs for jet lag, and we do not start them remotely for anything else.
They fragment sleep quality rather than improving it, leave next-day impairment, and carry real risk during long flights.
No — that is a different problem. Dehydration and cramped seating cause travel fatigue, which is real but separate.
Hydration helps you feel less rough; it does not move your body clock.
Do not adjust at all. Stay on home time, schedule meetings for hours that suit your home clock, and come back unshifted.
Two days is not enough time to adjust and then re-adjust, and attempting it leaves you worse off at both ends.
Plan it before you fly, not at the airport. Insulin, the contraceptive pill, anticoagulants, epilepsy and thyroid medication all need a specific approach.
This is the part of jet lag with real consequences, and it is worth a consultation.
Not if no time zones were crossed — that is travel fatigue, and it clears in a day or two.
And if you have a fever, it is neither. Fever after travel needs assessment, urgently if you have been anywhere with malaria.
August 30, 2026
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