Anti-jet-lag apps promise to get you back on track in 24 hours. Your internal clock only shifts by about one hour a day, and light at the wrong time can even push it in the wrong direction.
Jet lag is a temporary mismatch between your circadian clock and the destination’s social schedule, often compounded by sleep debt, overnight flights, noise, alcohol and inactivity. Light is the main lever for resetting the clock, but its effect depends on biological time. Exposure at the wrong time can shift your rhythm in the opposite direction.
A serious protocol therefore does not promise adaptation in twenty-four hours. It combines preparation, planned sleep, light and darkness, reserving melatonin for situations where its timing and safety are understood.
Why travel direction matters
Travelling east generally means falling asleep and waking earlier according to your old clock, requiring a phase advance. Travelling west often requires a phase delay. Because the human clock tends to run slightly longer than twenty-four hours, delaying is often easier than advancing, though this is not true for everyone.
The number of time zones is not enough. Paris to New York, Paris to Tokyo and a flight arriving at 5 am do not create the same light-exposure windows. Your usual sleep schedule, chronotype, the season and available light all affect the plan.
Symptoms can include insomnia on the new schedule, daytime sleepiness, reduced concentration, digestive problems and altered mood. They do not prove a “metabolic fracture” or mitochondrial damage. They mainly result from misalignment and disrupted sleep.
Light is not a universal dose of lux
The retina contains light-sensitive cells that project to the suprachiasmatic nucleus. Intensity, spectrum, duration and previous light exposure all influence the response. Outdoor light is often far more intense than indoor lighting, even on overcast days.
But a chart saying “below 50 lux equals maximum melatonin, above 10,000 lux equals total suppression” would be misleading. The response varies with timing, pupil size, previous exposure and the individual. Lux measures light weighted for human vision, not its circadian effect directly.
The phase response curve provides the essential principle:
- light received after the body temperature minimum, often towards the end of the biological night, tends to shift the clock earlier
- light received before this minimum, during the biological evening or early night, tends to shift it later
- near the tipping point, an incorrect estimate can reverse the intended effect
That is why “bright light as soon as you land after flying east” is not always safe after a large time shift. If your body still thinks it is the middle of the night, that light can delay adaptation.
Prepare for the trip without turning your week upside down
For a very short stay, full adaptation may not be desirable. If appointments can stay close to your home schedule, partially maintaining your old rhythm sometimes reduces the burden. For a longer stay, start two to three days beforehand if your schedule allows.
Heading east, gradually move bedtime, waking, meals and morning light earlier by thirty to sixty minutes a day. Reduce evening light. Heading west, gradually move these cues later and seek more light towards the end of the day.
Preparation must still allow enough sleep. Getting up two hours earlier without actually falling asleep earlier adds sleep deprivation that can be more impairing than the time shift.
Prepare for the practical constraints too:
- local arrival time and a realistic bedtime
- access to outdoor light or a device designed for this purpose
- eye mask, earplugs and comfortable clothing
- medicines stored as instructed, especially insulin and treatments with strict dosing schedules
- a medical appointment before departure if a treatment schedule needs to shift across time zones
During the flight: reduce sleep debt, not “toxins”
Set your watch to destination time for planning, but do not force yourself to sleep if the target schedule clashes with your biology. Use an eye mask and earplugs during the destination’s night, and light during its day when your phase-shifting plan calls for it.
Eat according to your hunger. Animal studies show that meal timing influences peripheral clocks, but human evidence does not support a water-only fast throughout the flight as a jet lag treatment. This practice is unsuitable for some people who have diabetes, are pregnant, older or adolescent, have an eating disorder or take medicines with meals.
Limit alcohol, which fragments sleep and causes dehydration. Caffeine can support alertness at a targeted time, but taken close to planned sleep it perpetuates insomnia. Use the caffeine calculator as an estimate, keeping individual variability in mind.
Move regularly in line with safety instructions and hydrate normally. Compression garments do not “drain” metabolic waste. They may be indicated for venous risk in some travellers, following professional advice and with the correct fit.
On arrival: establish cues without getting light at the wrong time
The local day becomes your behavioural reference, but adapt light exposure to the direction and size of the time shift.
For a moderate eastward shift, generally seek morning light after your target waking time and reduce it in the evening. For a moderate westward shift, generally seek light in the late afternoon or early evening and avoid very early morning light if it shifts the clock earlier.
After crossing eight or more time zones, these simple rules can land you on the wrong part of the curve. Use a validated jet lag calculator, a sleep specialist or a plan based on your usual schedule and exact route. Opaque glasses or an eye mask can help you avoid a light-exposure window, but orange-tinted glasses are not “mitochondrial” protection.
The other time cues are simpler:
- eat meals at local times without overeating or forced fasting
- do some light outdoor activity during the appropriate light-exposure window
- keep a regular local waking time
- avoid a long, late nap
- if safety requires it, take a short, planned nap rather than fighting sleep at the wheel
The recommendation to “never nap” is dangerous if you need to drive or operate machinery. A twenty- to thirty-minute nap can reduce sleepiness, even though it does not reset the clock. Allow time for sleep inertia to wear off before a safety-critical task.
Melatonin: useful, but highly timing-dependent
Melatonin is both a circadian signal and a sleep-promoting substance. A Cochrane review of ten trials in travellers concluded that it reduced jet lag when taken near the target bedtime, generally between 10 pm and midnight at the destination, after crossing five or more time zones. Daily doses of 0.5–5 mg had broadly similar effectiveness, while 5 mg shortened the time to fall asleep more than 0.5 mg. Doses above 5 mg provided no additional benefit.
These findings come from limited trials and do not make 0.5 mg an “optimal” dose for everyone. Taken too early in the day, melatonin can cause sleepiness and delay adaptation. Immediate-release and prolonged-release formulations are not equivalent.
Before taking it, check with a doctor or pharmacist if you are pregnant or breastfeeding, under eighteen, have epilepsy, an autoimmune condition, liver or kidney disease, or take anticoagulants, sedatives or other medicines. Do not combine melatonin and alcohol. Do not drive after taking it. Product quality and regulatory status vary by country.
Temperature and sleep at the hotel
An uncomfortably warm room can increase wakefulness, but cooling it to 18 °C does not directly speed up your body clock’s adjustment. Temperature supports sleep; it is not a strong enough zeitgeber to replace light.
A warm shower one to two hours before bed may shorten the time it takes some people to fall asleep, according to a separate meta-analysis. Use it for comfort if you tolerate it. It does not “trigger” slow waves or undo poorly timed light exposure. Our guide to body temperature and sleep details the precautions.
Compact protocol by direction
For an eastbound trip lasting several days, follow this framework:
- before departure, gradually move sleep and morning light earlier
- during the flight, protect the destination’s night and avoid alcohol
- on arrival, use carefully timed morning light, not automatic exposure after a large time shift
- reduce evening light and keep a stable local waking time
- consider melatonin near local bedtime only after checking safety
For a westbound trip, adapt as follows:
- before departure, gradually shift your schedule later if possible
- seek light towards the end of the local day
- limit very early morning light at first if it shifts the clock earlier in the wrong direction
- take a short nap if sleepiness compromises safety
- keep meals and activity on local time
Reassess after three days. If the plan noticeably worsens insomnia, stop improvised artificial light exposure and seek advice. Eye pain, manic symptoms, a triggered migraine or retinal disease warrant particular caution with light therapy.
The Verdict
Light is the best circadian lever for jet lag, but “morning heading east, evening heading west” is only a first approximation. The larger the time shift, the greater the risk of poorly timed light.
Prepare gradually, protect sleep duration, eat without forced fasting and treat melatonin as a chronobiological intervention whose timing matters. The realistic goal is to reduce symptoms and lapses in alertness, not force every clock in your body to synchronise in a day.
