Jet lag happens because your internal body clock stays on home time while the world around you has jumped to a new one. Your body doesn't instantly reset — it drifts back into sync at roughly one time zone per day. And there's a genuine asymmetry: flying east is harder than flying west, because it asks your body to do the thing it's worst at — going to sleep earlier than it wants to.
Your body runs on a 24-hour clock
Deep in the brain sits the suprachiasmatic nucleus — a master clock that runs your circadian rhythm: the daily cycle of sleepiness, alertness, hormone release, body temperature and digestion. Left alone it runs close to, but not exactly, 24 hours, so every day it resets using external cues. The most powerful cue by far is light; meal times and activity matter too. Cross several time zones in a few hours and your clock is suddenly hours out of step with the local sun — that mismatch is jet lag.
Why it makes you feel so rough
Because that clock controls more than sleep. When it's misaligned, different body systems disagree about the time:
- Sleep — wide awake at 3am, wiped out at midday
- Digestion — your gut expects meals on home schedule, causing appetite chaos, nausea or constipation
- Alertness and mood — brain fog, irritability, poor concentration
- Physical — headaches and general malaise, partly dehydration from the flight itself
Rule of thumb: jet lag becomes noticeable crossing three or more time zones, and worsens with each additional zone.
Why east is worse than west
Here's the core asymmetry. Your natural clock runs slightly longer than 24 hours, so lengthening your day comes naturally — which is exactly what flying west asks (you stay up later, sleep later; London to New York, you just push bedtime back). Flying east demands the opposite: London to Tokyo, you must fall asleep and wake up earlier than your body is ready for — and forcing sleep before your clock is tired is far harder than staying up a bit longer. Same number of zones, tougher direction.
This is why the direction you travel changes the whole strategy — including when to seek light and, if you use it, when to take melatonin. Our guide on timing melatonin for your direction of travel covers that precisely.
How long does it last?
The usual estimate is about one day of recovery per time zone crossed, though it varies with direction (add time going east), age, and how much you help the adjustment along. A five-hour eastward trip might mean the best part of a week feeling off; a westward one of the same distance, rather less. The good news is that light exposure, meal timing and sleep habits can meaningfully speed the reset — jet lag isn't purely something to endure.
What makes it worse
- Dehydration — dry cabin air plus too little water amplifies the fatigue and headache
- Alcohol and excess caffeine — both disrupt already-fragile sleep on and after the flight
- Poor flight sleep — arriving with a sleep debt on top of the time shift
- Fighting local time — napping at the wrong moments and eating on home schedule both slow the reset
The bottom line
Jet lag is a mismatch between your body clock and local time that resolves at roughly a zone a day, worse heading east because your body resists early sleep. Understanding the mechanism is what makes the fixes make sense: they all work by dragging your clock toward local time faster — with light, timing, and sometimes melatonin. Our guides on beating jet lag without pills and how melatonin helps cover the how.
Frequently asked questions
Why is flying east worse for jet lag?
Your body clock naturally runs slightly longer than 24 hours, so extending your day (flying west) is easier than shortening it (flying east). Eastward travel forces earlier sleep and waking, which the body resists.
How many time zones before you get jet lag?
Symptoms typically start around three or more zones and intensify with each additional one. One or two zones rarely cause meaningful jet lag.
How long does jet lag last?
Roughly a day per time zone crossed, often longer eastward. Light exposure, meal timing and good sleep habits can shorten it.
Does jet lag get worse with age?
Many people find recovery slower as they get older, though it's individual. The same strategies help at any age.
References
- Herxheimer A, Waterhouse J. The prevention and treatment of jet lag. BMJ. 2003;326(7384):296–297.
- Eastman CI, Burgess HJ. How to travel the world without jet lag. Sleep Med Clin. 2009;4(2):241–255.
- NHS. Jet lag. nhs.uk/conditions/jet-lag.
- Roenneberg T, Merrow M. The circadian clock and human health. Curr Biol. 2016.