How long does jet lag last, and why is flying east harder?
Short answer: Jet lag usually eases within a few days. On average the body clock catches up by about an hour a day after flying east and about an hour and a half a day after flying west, so six time zones eastward can take close to a week. East is harder because the body clock runs slightly longer than 24 hours, so it moves later more easily than earlier.
How long does jet lag last?
Usually a few days, and longer the more time zones you cross. CDC travel guidance gives an average adjustment rate of 1 hour a day after eastward flights and 1.5 hours a day after westward flights. That matches the familiar rule of about a day per time zone for trips east, and a little less for trips west.
Early estimates from real travellers point the same way: about 57 minutes a day after eastward flights and 92 minutes a day after westward ones. After big shifts, full adjustment can take a week or more.
Why is jet lag worse going east?
Flying east asks your body clock to move earlier, and that is the harder direction. In a carefully controlled lab study, the human body clock ran on an average cycle of 24.18 hours, slightly longer than a day. So most people already tend to drift a little later each day, which makes delaying the clock easier than advancing it.
Going west asks you to stay up later, which works with that drift. The US National Heart, Lung, and Blood Institute notes that jet lag disorder is often more severe after travel east than after travel west.
Is jet lag real, and what does it do to your body?
Yes. Jet lag disorder is a recognised circadian rhythm sleep disorder that can follow travel across at least two time zones. It happens because your internal clock is still on home time while the day around you has moved, and that clock times sleep, hormone release, digestion and body temperature.
The NHS lists difficulty sleeping at bedtime and waking up in the morning, tiredness, trouble staying awake during the day, poor sleep quality, and problems with concentration and memory. Jet lag can also bring indigestion, nausea, constipation, changes in appetite and mild anxiety.
How does light help you adjust?
Light and dark have the biggest influence on the body clock, so timing matters. Bright light in the morning helps move the clock earlier, while light in the evening and early night moves it later. After you land, get outside in daylight at your destination; both the NHS and the CDC advise this.
For a short trip of 2 to 3 days, the NHS suggests keeping your home eating and sleeping times, so you do not have to shift twice. Melatonin rules differ by country, and the NHS does not recommend melatonin tablets for jet lag, so ask a doctor or pharmacist before using it. For more on moving the clock, see how to reset your circadian rhythm.
Why can jet lag feel worse coming home?
Because direction matters, not which way is home. If your outbound trip went west, the flight home goes east and asks the clock to advance, the harder direction. In one worked example of a round trip from San Francisco to Beijing, the body clock took about 5 days to settle after flying west, but about 9 days after the flight home to the east.
Shift work creates a similar mismatch between the body clock and the hours you keep, as explained in shift work and your body clock.
How Actogram looks at this
Actogram is a MedTech company whose focus is the body clock. It reads the daily rhythm of rest and activity from ordinary minute-level step counts from a phone or watch. It aims to work for irregular days, such as shift work, jet lag and fragmented days, and it is designed for live data that keeps coming in, not only for finished recordings. Today its tools are for research and wellness use and do not diagnose, treat or prevent any condition.
Sources
- Jet Lag Disorder, CDC Yellow Book, 2025
- Jet lag, NHS, 2023
- Circadian Rhythm Disorders: Types, National Heart, Lung, and Blood Institute, 2022
- How To Travel the World Without Jet lag, Sleep Medicine Clinics, 2009
- Stability, precision, and near-24-hour period of the human circadian pacemaker, Science, 1999
- Circadian Rhythms, National Institute of General Medical Sciences, 2023
General information, not medical advice. If sleep or tiredness problems affect your daily life, talk to a doctor. Reviewed 6 November 2026 by the Actogram team.