Jet lag — what it is and how long it lasts
Updated 16 May 2026
1 · What this actually is
Short answer
Transient circadian misalignment after crossing time zones — your internal clock is still set to origin time while you're living on destination time. Recovery is roughly 1 day per timezone eastward and 1.5 days per timezone westward if you do nothing to help.
2 · Most likely causes
- 1
Circadian-clock mismatch
Your suprachiasmatic nucleus shifts about 1 hour per day on its own. Cross more zones than that and you spend several days misaligned.
- 2
Direction of travel
Eastward travel forces a phase advance (shorter day). Westward forces a phase delay (longer day). The clock advances more reluctantly than it delays — so eastward is harder.
- 3
Time zone count
Linear effect roughly. 3 zones is a few days of mild symptoms. 9 zones is a week or more of substantial disruption.
- 4
Light exposure at wrong times
Most people exacerbate jet lag by getting bright light at the wrong times — sleeping in a dark hotel during the destination's morning, or staying up under bright light when they should be sleeping.
3 · What the evidence says works
Timed bright light exposure
The most powerful intervention. After eastward travel: morning light at destination, avoid evening light. After westward: evening light, avoid early-morning light. See the jet-lag calculator for a specific schedule.
Evidence: strong
Low-dose melatonin (0.3–0.5 mg) at destination bedtime
Particularly useful eastward. Take for 3–5 nights. Higher doses don't work better.
Evidence: strong
Eat meals at destination times
Food is a secondary zeitgeber. Eating at local times speeds peripheral clock adjustment even when you're not hungry.
Evidence: moderate
Pre-shift before travel
Start shifting bed/wake times 1–2 hours per day toward destination time for several days pre-flight. Reduces jet lag substantially for trips of 6+ zones.
Evidence: moderate
Strategic short naps
Brief 20-minute naps at destination can manage acute sleepiness without preventing nighttime sleep. Avoid 2+ hour naps until adjusted.
Evidence: moderate
4 · What doesn't work
Common claims, ranked by reality
Claim
Sleep on the flight to land 'in sync'.
Reality
Sleep on a 6-hour flight rarely matches the timing you need at destination. Better to plan sleep around the destination schedule, not the flight.
Claim
More melatonin works better.
Reality
0.3–0.5 mg works as well or better than 3–5 mg for phase shifting. Higher doses produce longer half-lives and residual grogginess without bigger phase shift.
Claim
Alcohol on the flight helps you sleep.
Reality
Alcohol fragments sleep, dehydrates you, and worsens recovery on landing. Avoid.
Claim
Jet lag isn't a real thing — just adjust.
Reality
Jet lag is well-documented in physiological measurements (cortisol, melatonin, body temperature) — not just subjective. You can't reason your way through it.
5 · When to see a doctor
Book an appointment with a GP — and consider asking about a sleep study — if any of these apply:
- Symptoms persist longer than expected for your timezone count + recovery rate.
- You travel frequently across multiple zones and develop chronic insomnia or daytime sleepiness.
- You take medication where timing matters (insulin, antiarrhythmics, immunosuppressants) — ask before travel.
- Pre-existing sleep disorder is worsening with travel.
Common follow-up questions
- Why is eastward jet lag worse?
- The human circadian rhythm naturally runs slightly longer than 24 hours, so delays (westward travel) feel more comfortable than advances (eastward). Phase advance is harder for the SCN to accomplish.
- Should I take melatonin for jet lag?
- For travel of 5+ zones eastward, yes — low dose (0.3–0.5 mg) at destination bedtime for 3–5 nights. Westward travel rarely benefits as much.