What jet lag is
Jet lag happens when rapid travel across time zones leaves the body's daily timing out of step with the destination. That daily timing is the body clock, also called the circadian rhythm. A traveller may struggle to sleep during the local night or stay alert during the local day. Symptoms can also include poor concentration and stomach disturbances. Jet lag is more specific than simply being tired after a journey. [1]
Sleep lost while travelling can add to the problem. Recovering that sleep and shifting the clock are related tasks, but they are not the same task. A traveller can need more sleep even when the timing plan is sensible. Equally, feeling sleepy after a product does not show that the body clock has moved to the right time. [1]
Keep these questions separate
- Sleep opportunity
- Enough sleep?How much sleep was lost before or during the journey?
- Clock timing
- Which direction?Does the body need to move earlier or later?
- Daytime function
- Alert enough?How do symptoms affect the day's activities and safety?
Jet lag planning involves the route, internal timing and symptoms. This is an explanation, not a diagnostic questionnaire. [1]
This page covers adult time-zone travel. A long-standing late sleep pattern and repeated night-shift work need different assessment and treatment questions. The main sleep guide helps distinguish those problems, and the sleep collection gathers the related explanations.
Why eastward and westward travel differ
Eastward travel usually requires the body clock to move earlier; westward travel usually requires it to move later. Researchers call moving earlier an advance and moving later a delay. These words describe the internal rhythm, not whether the flight or destination clock is late. Most people find delaying the rhythm easier than advancing it. [1]
The usual direction of adjustment
| The question | Eastward journey | Westward journey |
|---|---|---|
| What happened to local clock time? | The destination is generally ahead of home. | The destination is generally behind home. |
| How does the body usually need to change? | Sleep and wake earlier in its internal cycle. | Sleep and wake later in its internal cycle. |
| What is the timing term? | Advance: move earlier. | Delay: move later. |
| Is this a light or melatonin schedule? | No. Direction is only part of the timing calculation. | No. Route, time zones and the person's timing still matter. |
This describes the usual adjustment direction. It does not resolve large shifts or provide instructions for a particular flight. [1]
An eastward destination can be ready for bed while the traveller's body still treats it as earlier in the day. The problem is not necessarily an inability to sleep under any circumstances: the available bedtime and the biological night may disagree. That distinction also matters outside travel, as the sleep overview explains.
What to plan before, during and after travel
A useful plan protects sleep and considers how the destination schedule differs from home. CDC travel guidance describes adjusting sleep towards the destination before departure, timing activities during the flight and managing sleep and alertness after arrival. Its examples are starting points for a route-specific discussion, not one timetable for every traveller. [1]
- 1
Before departure
Consider the destination schedule and whether sleep can move towards it for a few days beforehand. Include the flight and arrival time, rather than planning only the first destination bedtime.
- 2
During the journey
Consider destination time when organising sleep and other activities. Preserve the chance to sleep; alcohol and caffeine are relevant to the quality of that opportunity.
- 3
After arrival
Plan the local sleep period and daytime activities together. Short naps can help alertness, while a long daytime sleep may make the next night harder.
For example, CDC describes moving sleep an hour earlier for eastward travel, or later for westward travel, on each of the two or three days before a trip when feasible. This is an illustrative preparatory strategy. It is not a reason to create more sleep loss by forcing an unrealistic bedtime or keeping a dangerously tired person awake. [1]
The length of the stay also changes the planning question. CDC guidance includes maintaining home-base sleep hours on very short trips where adaptation would be limited. A brief stay and a longer visit therefore need not aim at the same degree of clock adjustment. [1]
Why morning light is not one universal rule
Light can help shift the body clock, but its effect depends on when the body receives it in its internal cycle. “Morning” on a destination clock may not yet be biological morning for the traveller. Advice that ignores that difference can move timing in the wrong direction. [1]
CDC explains that light after the low point of the body's circadian cycle tends to move timing earlier, while evening light generally moves it later. The low point is related to the habitual sleep pattern, and appropriate exposure depends on the direction and size of the journey. These principles explain the limits of generic advice; they do not provide enough information to calculate a personal exposure window here. [1]
The same word can therefore describe two different things:
- Destination time
- The local clockWhat the clock at your destination says.
- Body-clock time
- Your internal rhythmThe phase your body is still following after the journey.
These can disagree after crossing time zones. Light timing needs to account for that difference.
This is a conceptual explanation, not a personalised phase diagram. The light and screens guide discusses ordinary light exposure; jet lag adds the route-specific mismatch. A darker or more comfortable bedroom can support the opportunity to sleep without proving that the body clock has shifted.
What the melatonin evidence can tell you
Melatonin can reduce jet lag symptoms in some travellers, but the evidence does not establish one optimum routine for all journeys. Melatonin is a hormone involved in daily timing. Taking it when the internal clock treats the time as morning can shift that clock later; taking it in the internal early evening can shift it earlier. Incorrect timing can increase the mismatch. [1, 2]
An older Cochrane review found symptom benefits in trials involving travel across several time zones. The results were mainly travellers' ratings of symptoms and sleep. The evidence is old, and review versions differ in the reported count of positive trials. The useful conclusion is the qualified benefit, not a success rate assembled from those counts. [2]
One 1998 trial compared fast-release melatonin, controlled-release melatonin and placebo after eastbound flights across six to eight time zones. Fast-release means the preparation releases its melatonin relatively quickly; controlled-release means release is spread over a longer period. The study favoured its fast-release preparations, but dose and release pattern differed together, so it cannot establish that every fast-release product is superior for every use. [3]
| Feature of that trial | What was actually studied |
|---|---|
| Journey | Eastbound travel across six to eight time zones |
| Preparations | 0.5 mg fast-release, 5 mg fast-release, 2 mg controlled-release or placebo |
| Timing and duration | At bedtime for four days after the flight |
| Participant accounting | 320 recruited; 234 were compliant and completed the study |
| Important limit | Self-reported outcomes; the accessible abstract does not give precise between-group effect sizes |
These are study conditions, not instructions for a trip. Funding and conflicts were not established from the accessible abstract. The review includes this trial, so citing both does not create two independent confirmations. [3, 2]
The melatonin profile separates these uses and formulations. The broader sleep-supplement comparison addresses a different question; chronic-insomnia evidence should not be treated as interchangeable with jet lag evidence.
Why melatonin pages give different dose instructions
Different numbers can come from different purposes and products. A research dose describes what a trial tested. Guidance for shifting the body clock describes a physiological use. An authorised product label describes one medicine in one jurisdiction. Choosing the highest or lowest number from those sources does not produce a personal recommendation.
Three sources of numbers, three different meanings
| Instruction source | What it describes | What it does not establish |
|---|---|---|
| The 1998 trial | Its 0.5 mg, 5 mg and 2 mg study preparations and four postflight nights. | A universal dose, a current product recommendation or every route. |
| CDC clock-shift guidance | It discusses 0.5 to 1 mg as often sufficient for a circadian shift. | One destination-clock schedule that works for everyone. |
| One UK product label | Colonis 3 mg tablets: standard 3 mg daily for at most five days; conditional 6 mg option. | Worldwide availability or instructions for all melatonin products. |
This table compares provenance, not efficacy. The UK product is prescription-only, and its timing and precautions remain attached to that product. [3, 1, 4]
The UK label specifies the first dose on arrival at habitual bedtime and a destination-time window excluding use before 20:00 or after 04:00. It also contains food, alcohol, medical-condition and interaction precautions. Those directions are not a substitute for checking suitability or for route-specific clock-shift planning, and they must not be silently converted into international advice. [4]
How long adjustment takes, and how to stay alert
Adjustment is usually gradual, with large individual differences. CDC gives average adaptation rates of about one hour per day eastward and one and a half hours per day westward. These are rough guidance figures, not measurements of how long every person's symptoms last. [1]
Rough average clock adjustment in CDC guidance
Eastward travel
About 1hours per day
Westward travel
About 1.5hours per day
These are guidance averages, not results from one treatment trial. They describe clock adjustment rather than guaranteed symptom recovery. Dividing a journey's time-zone difference by a bar does not give a reliable personal recovery date. [1]
For daytime alertness, CDC discusses short naps of 20 to 30 minutes and warns that longer daytime naps can make subsequent night-time sleep harder. That is a travel strategy, not a universal nap rule for insomnia. Caffeine can support alertness, but taking it near the planned sleep period can work against the chance to sleep; dose also matters. The main sleep guide explains that trade-off. [1, 5, 6]
Sleep-promoting medicines also bring possible adverse effects and next-day impairment. CDC cautions against long-acting sedatives and sedating antihistamines in this context. A list of medicines used during travel is not a menu for choosing a pill on your own, and this guide does not provide a medicine schedule. [1]
When the next step is assessment
Persistent difficulty, substantial daytime sleepiness or symptoms that do not fit the journey deserve assessment. Jet lag should not become an explanation for every bad night after returning home. Breathing pauses or gasping during sleep can point to a different problem, such as sleep apnoea. Being too sleepy for driving is a safety issue even if the likely cause is travel. [1, 7]
For a repeated travel pattern, a pre-travel discussion can consider the journey, work demands, usual sleep and product suitability together. For ongoing sleep difficulty away from travel, the guide to sleeping better explains opportunity, insomnia and the role of CBT-I. The melatonin profile is the next page for formulation and label questions.
Find the sleep question that fitsTravel timing, persistent insomnia, bedroom conditions and supplement evidence belong to different explanations.