Jet lag happens when your internal body clock is still running on the time zone you left, while the world around you runs on a new one. The most reliable way to beat it is to shift your clock deliberately — using bright light at the right hours, darkness at the wrong ones, and small doses of melatonin timed to your destination’s evening. Most people re-adjust at a rate of roughly one time zone per day, and eastward travel takes longer than westward (Waterhouse, Reilly, Atkinson & Edwards, 2007, The Lancet).
That rate isn’t a personal failing. It’s the physics of a biological oscillator being asked to move. Knowing that in advance changes what you expect from yourself on day two of a trip.
What actually causes jet lag?
Your sleep timing is governed by a master clock in the brain called the suprachiasmatic nucleus, which stays locked to the outside world mainly through light hitting your retina. Fly across six time zones and that clock doesn’t move with the plane. It keeps releasing melatonin, dropping your core body temperature, and raising cortisol on the old schedule — so you feel wide awake at 2am local time and foggy at noon.
The classic symptoms follow from that mismatch: difficulty falling asleep or staying asleep, daytime sleepiness, poor concentration, low mood, and digestive upset. The digestive part is often the surprise, and it has a real basis — your gut and liver run their own clocks, and they don’t re-sync at the same speed as your brain does. If the underlying machinery is unfamiliar, how your circadian rhythm works is the fuller explanation.
It’s also worth separating jet lag from travel fatigue, which is the exhaustion of a long journey itself: dehydration, cramped seating, disrupted meals, airport stress. Travel fatigue resolves after a night or two of decent rest. Jet lag doesn’t, because the clock hasn’t moved yet.
Why is flying east so much harder than flying west?
Left with no time cues at all, the average human clock runs slightly longer than 24 hours. That makes it naturally easier to stretch your day than to shorten it. Flying west asks you to stay up later — a phase delay, which your clock does willingly. Flying east asks you to fall asleep earlier and wake earlier, a phase advance, which it resists.
The practical version: after a westward flight you might re-adjust at close to 1.5 time zones per day; after an eastward flight of the same distance, closer to one zone per day or less. A six-hour eastward trip can therefore take the better part of a week to fully settle (Waterhouse et al., 2007, The Lancet).
How do you use light to shift your body clock?
Light is by far the strongest tool you have, and its effect depends entirely on when you get it. Researchers have mapped this precisely with what’s called a phase response curve: light in the hours after your body’s temperature minimum — roughly the last part of your usual night and early morning — advances your clock, while light in the hours before it delays your clock (Khalsa, Jewett, Cajochen & Czeisler, 2003, The Journal of Physiology).
Translated into travel advice:
- Flying east (you need to advance): seek bright light in the destination’s morning, and avoid bright light in the early morning hours before your body thinks it’s morning — which for the first day or two may mean wearing sunglasses outdoors until mid-morning. Getting this backwards is the single most common way people accidentally make eastward jet lag worse.
- Flying west (you need to delay): seek bright light in the destination’s late afternoon and evening, and keep the early morning dim.
Outdoor daylight is dramatically brighter than any indoor lighting, so a walk beats sitting near a window. This is the same lever behind morning sunlight and sleep, just aimed at a moving target.
Shifting your schedule by an hour a day in the days before you fly — going to bed and getting up earlier for an eastward trip, later for a westward one, with light timed to match — meaningfully reduces how much adjusting is left to do on arrival (Eastman & Burgess, 2009, Sleep Medicine Clinics). It’s rarely realistic to do this fully, but even two partial days helps.
Does melatonin help with jet lag?
This is one of the few sleep supplements with genuinely strong evidence for a specific use. A Cochrane review of ten placebo-controlled trials found that melatonin taken close to the target bedtime at the destination reduced jet lag after flights crossing five or more time zones, with eight of the ten trials showing benefit. Doses between 0.5mg and 5mg were similarly effective for the clock-shifting effect, and doses above 5mg were no better (Herxheimer & Petrie, 2002, Cochrane Database of Systematic Reviews).
The important nuance is that melatonin here is working as a timing signal, not a sedative. Taken at the wrong hour it can shift your clock in the wrong direction. That’s why the timing instruction — destination bedtime, not “whenever you feel tired” — matters more than the dose. For the broader picture of what melatonin does and doesn’t do outside of travel, see does melatonin actually help you sleep.
Melatonin is regulated as a supplement in the US, so actual content per tablet varies. It also interacts with some medications, and it isn’t well studied in pregnancy or while breastfeeding — worth a conversation with your clinician rather than a guess.
What else moves the clock — meals, exercise, caffeine?
Light is the headline, but it isn’t the only input.
Meal timing shifts your peripheral clocks. In a controlled laboratory study, delaying meals by five hours delayed participants’ blood glucose rhythm by nearly the same amount and shifted clock gene expression in fat tissue, without moving the brain’s master clock (Wehrens, Christou, Isherwood et al., 2017, Current Biology). Eating on local time from the first day gives your body a second, consistent signal.
Exercise has its own phase response curve. Morning and afternoon exercise tends to advance the clock, while late-evening exercise tends to delay it (Youngstedt, Elliott & Kripke, 2019, The Journal of Physiology). A daylight walk on arrival day is doing double duty — light and movement pointing the same way.
Caffeine helps you cope with daytime sleepiness but doesn’t move your clock, and it lingers longer than most people expect. Using it after local midday can quietly sabotage the night you’re trying to build.
How does jet lag affect new parents?
Travelling with a baby or toddler stacks two clock problems on top of each other. Your child’s rhythm has to shift too, and their sleep is far less flexible than yours — which means the wake windows that used to work land in the wrong places for several days. Many parents find the child’s night wakings anchor them to the old time zone longer than they’d re-adjust alone.
A few things that reduce the strain: put everyone on local mealtimes and local daylight immediately rather than easing in, accept that the first two nights will be fragmented, and treat the trip as a temporary sleep debt rather than a schedule failure. Naps are legitimate here — a short early-afternoon nap in the new time zone takes the edge off without much cost to that night’s sleep, while a three-hour late-afternoon nap will hold your clock in place. The timing rules in how to nap apply cleanly to travel.
If you’re breastfeeding, night feeds will keep happening on your body’s schedule for a while regardless of what the clock on the wall says. That’s expected, not a sign the adjustment isn’t working.
How does jet lag affect perimenopausal women?
Circadian rhythms change with age. Older adults tend toward earlier timing, produce a lower-amplitude melatonin signal, and generally show reduced tolerance for abrupt schedule shifts — including shift work and transmeridian travel (Duffy, Zitting & Chinoy, 2015, Sleep Medicine Clinics). If travel used to cost you a day and now costs you four, that shift is real and well described in the literature. The broader version of this pattern is covered in why sleep gets worse with age.
Fluctuating estrogen and progesterone add a second layer. Night sweats and hot flashes already fragment sleep for many women in perimenopause, and a body clock that’s out of position tends to place the vulnerable early-morning hours right where the disruption lands. Hotel rooms are usually warmer than bedrooms at home, which doesn’t help — asking for a fan or turning the temperature down before you unpack is a small, unglamorous intervention that does more than most supplements. If that’s your pattern, perimenopause and sleep and night sweats go deeper.
What should you actually do on arrival day?
- Set your watch and your eating to local time as soon as you board.
- Get outdoors in daylight at the right end of the day for your direction of travel — morning for eastward, late afternoon for westward.
- If it’s a short trip of one or two days, consider not adjusting at all and staying on home time where the schedule allows. Shifting twice in three days is often worse than not shifting once.
- Keep the first night’s expectations low. Broken sleep on night one is the normal course, not a sign that the plan failed.
- Give it roughly a day per time zone before judging how you’re doing. When you get home, the same rules apply in reverse — see how to fix your sleep schedule.
The calm version
Jet lag is a timing problem, not a character problem, and it resolves on a predictable schedule whether or not you optimise it perfectly. Light in the right window, meals on local time, and a bit of patience will get you most of the way there. If you’re travelling with a baby or through a hormonal transition, expect the adjustment to take longer than it used to — that’s biology, not backsliding. Mendtide watches how your sleep re-settles across a trip and points out what’s actually shifting, without turning a rough night into a verdict.
Your clock will get there. It just moves about an hour a day, and no amount of effort makes it move faster.