The most-repeated rule about jet lag is “one day per time zone”. It is the answer pilots quote, the answer travel guides print, the answer your colleague gives when you arrive groggy on Monday. It is also wrong in a way that matters.
Without intervention, the human circadian system shifts about 0.5 to 1 hour per day. That is the source of the one-day-per-time-zone rule. It is also the unaided rate, which is to say the rate your body achieves if you do nothing on purpose. With well-timed light, melatonin, sleep, food, and exercise, the daily shift can be roughly doubled. The eastward direction is harder than westward. And the recovery window is longer than most travellers track, because the symptoms keep going long after the obvious fatigue has lifted.
This article walks through what the research actually says, what your body is doing under the hood, and what you can do about it on your next long-haul.
What jet lag actually is
Jet lag is what happens when your body’s internal clock and your local environment disagree. The clock lives in a tiny brain region called the suprachiasmatic nucleus, just above where the optic nerves cross, and it runs on a near-24-hour cycle that resets daily based on signals from the environment. Light is the strongest of those signals. Meal timing and exercise are weaker but real.
When you fly across time zones, you change your environment in a few hours. Your clock, which evolved against a planet that does not have airplanes, takes longer to catch up. The mismatch is what you feel as jet lag: poor sleep, brain fog, gut disruption, depressed mood, weaker workouts, slower reaction time. The deeper consequence is that every day of mismatch is a sub-standard day, even after the obvious sleepiness has lifted.
The 0.5 to 1 hour per day rule
Several decades of laboratory research support the idea that the circadian system, given no special intervention, can shift roughly half an hour to an hour per day. The exact number depends on direction, prior light exposure, age, chronotype, and whether the person is in a phase-advance or phase-delay scenario. The rule of thumb works as an order of magnitude, not a precise number.
A few specifics worth knowing:
- Westward travel (delays the clock) is biologically easier. Your body has to stay awake later than usual, which is more in line with the natural drift of an un-zeitgebered clock. The free-running circadian period in humans averages slightly longer than 24 hours.
- Eastward travel (advances the clock) is harder. You have to fall asleep and wake up earlier than your body wants. The same biology that makes Mondays painful in domestic life makes the Tokyo flight from New York objectively worse than the New York flight back.
- The half-time between when you cross and when you are fully adapted depends on the size of the shift. A two-hour shift can clear in two to three days unaided. A nine-hour shift takes nine days or more unaided, which is most of a week of impaired work.
The reason a circadian protocol is useful is that all of those numbers can move. Light, the strongest cue we have, can shift the clock by one to two hours per day if timed correctly. Mistimed, the same light makes the lag worse, which is why “stay awake until destination bedtime” is dangerous advice.
What the levers actually do
A modern jet-lag protocol pulls four scientifically-supported levers. Two are settled science. Two are well-supported but more modest.
Core levers (settled science)
Light. The body has a phase-response curve to light. Light in the morning, after your core body temperature minimum, advances the clock. Light in the evening, before that minimum, delays it. The minimum, sometimes called CBTmin, is the reference point everything is timed relative to. Bright outdoor daylight delivers enough lux to shift the clock meaningfully. Indoor light is usually too dim to count.
Sleep timing. Sleep is partly homeostatic (the longer you have been awake, the more you need it) and partly circadian (your body permits sleep in certain biological windows and resists it in others). Whether to sleep on the plane depends on which side of the biological day you are crossing. A good protocol tells you to sleep when sleep is biologically available, even if that means napping on a transpacific flight, and to stay awake when staying awake protects your destination night.
Caffeine. Caffeine is an adenosine-receptor antagonist. It works as an alertness lever within hours of consumption. It also has a half-life of about five to six hours, so timing matters: caffeine after about eight hours before bedtime in your destination zone interferes with the sleep window you are trying to protect.
Melatonin. Low-dose melatonin (0.3 to 1 mg) taken in the destination evening can advance the clock. Mistimed, it has no effect or backfires. Melatonin is a chronobiotic, not a sleeping pill, and the dose-timing window is narrow. We say a lot more about this on the science page.
Supportive levers (well-supported)
Exercise. Exercise has been shown to act as an independent phase-shifting zeitgeber. The effect is smaller than light, but additive. A workout placed at the right time of the destination day strengthens the rest of the protocol.
Meal timing. Peripheral oscillators (gut, liver) respond to meal timing. Aligning meals to destination clock time helps the rest of the body catch up to the central clock in the brain.
Hydration and electrolytes. A long-haul flight can cost two to three litres of fluid through respiration and cabin air. The dehydration alone produces fatigue and headache that look like jet lag and stack on top of it.
What jet lag actually feels like, day by day
A west-to-east trip from New York to Tokyo crosses 13 time zones (effectively 11 if you account for direction). The unaided pattern, day by day, looks something like this:
- Day 1: strong sleepiness in the late afternoon, struggle to fall asleep at destination bedtime, wake at 03:00 local
- Day 2: similar pattern, slightly milder
- Day 3 to 5: the obvious tiredness recedes; reaction time, mood, and focus are still measurably worse than baseline
- Day 6 to 9: subtle symptoms persist, especially in the late afternoon
- Day 10+: clock fully aligned
The first three days are where most travellers think jet lag lives. The last six are where most of the productivity cost actually shows up, because the person feels approximately functional and is therefore not protecting themselves.
This is why a multi-day plan matters. A protocol that gets you through the first night is not enough.
What we can do about it
A few practical takeaways. None of these are new science; the value of an app is that it does the timing math for you and adapts to your trip.
- Find the direction. Westward is easier. Eastward needs a more aggressive light-and-melatonin plan.
- Find your reference point. Light timing is anchored on CBTmin, which moves with your habitual sleep schedule. The same light at 06:00 local time advances or delays depending on where your CBTmin sits.
- Plan the flight, too. Sleeping at the right time on the plane (and avoiding sleep at the wrong time) is one of the highest-leverage moves available. It does not have to be uninterrupted; it has to be biologically correct.
- Use multiple levers. Light alone is the strongest, but light plus melatonin plus exercise plus meal timing is meaningfully better than any single one.
- Hydrate ruthlessly. Two to three litres of water through the flight, electrolytes added on the longer hauls.
- Protect the destination night. Caffeine before about eight hours before destination bedtime. Light limited in the late destination evening on the first day. A dark, cool, quiet sleeping environment.
Most jet-lag apps focus on light and melatonin. Circa runs the whole plan, with the levers above and a few more, and personalises every recommendation to your wake time, your office hours, and the realities of your trip. The first plan is free. Try it and see.
References: AASM circadian rhythm review (Frontiers, 2019); CDC Yellow Book, Jet Lag Disorder; PMC systematic review on travel and circadian rhythm (2024).