Jet lag and crew fatigue: circadian rhythm

A 46-year-old captain returns from a series of long flights. On the last one, she departed at night from a city in the Americas and landed in a European capital in the morning, after crossing six time zones eastward. She slept little during the rest break on board. At the hotel, she tried to go to bed at 11 pm local time, but stayed awake until almost 3 am. She woke up tired, with an upset stomach and difficulty concentrating at the afternoon meeting. On the way home, four days later, she tells the company doctor that she has been sleeping badly for two weeks and asks whether it is just jet lag or whether there is another problem. The scene is fictional and was written for teaching.

This text uses the picture as a scenario for clinical reasoning: what jet lag is, how to separate the misalignment of the biological clock from simple lack of sleep, which tests the guidelines indicate, which doses of melatonin and caffeine the sources describe and why the flight schedule enters the diagnosis. The text does not replace medical assessment or the rules of each country's aviation authority, and it is not a summary of regulations. If you play the case of the day on NEXO, you will recognize the challenge: two plausible explanations for the same tiredness and a clue about timing that decides between them.

Key points

• Jet lag is a circadian rhythm disorder caused by the misalignment between the biological clock and the time at the destination.
• Flying east is usually harder, because it requires moving the clock forward, and the human clock tends to run late.
• Accumulated sleep deprivation worsens performance even when the person does not notice the deficit.
• The sleep diary is the first recommended test; polysomnography is not routine.
• Light at the right time and melatonin near bedtime at the destination are the measures with the most support in the sources read.

Why jet lag is a reasoning scenario

Jet lag is a syndrome caused by disruption of the body clock. It affects most travelers who cross five or more time zones and is worse on eastward than westward flights (Herxheimer, 2014). Waterhouse and colleagues note that long flights bring two similar problems: travel fatigue and jet lag. The second is caused mainly by the body clock being at the wrong time in the new time zone, and the two have overlapping symptoms (Waterhouse and colleagues, 2007).

For those who work on board, the problem does not end with the trip. The flight schedule repeats, layovers are short and the body clock rarely manages to adjust to one time zone before the next. That is why the captain's case calls for two questions: is her clock out of place, or has she simply been sleeping little for weeks? In practice, the two usually go together.

What the scene teaches

The scene has five clues. The first is direction: six time zones eastward. The second is the type of insomnia: difficulty falling asleep at local time, typical of someone who needs to move the clock forward. The third is short sleep on board. The fourth is symptoms outside sleep, such as stomach discomfort and poor concentration. The fifth is time: two weeks of poor sleep, which go beyond the adjustment of a single trip. The post on signs, symptoms and history shows how the sequence of times guides the reasoning.

Pathophysiology: the clock that does not keep up with the plane

The jet lag chapter of the CDC travel guide, the Yellow Book, explains that the human circadian system has an intrinsic period slightly longer than 24 hours. That is why the body clock adjusts more easily to delays than to advances. Natural adaptation is about 1.5 hours per day after westward flights and about 1 hour per day after eastward flights (Riedy and Williams, CDC Yellow Book). In the captain's case, six time zones eastward mean almost a week for the clock to reach European time, and she stayed only four days.

Burgess and colleagues sum up the same idea: because the human clock delays more easily than it advances, traveling east is often harder than traveling west (Burgess and colleagues, 2003). In pilots, Sun and colleagues found slightly worse results on eastbound flights than on westbound flights (Sun and colleagues, 2024).

Light as the main signal of the clock

Light is the strongest signal for setting the clock, but the effect depends on the time of day. The CDC describes that morning light, received after the lowest point of body temperature, which usually falls two to four hours before the usual wake-up time, advances the clock. Evening light, before that point, delays the clock (Riedy and Williams, CDC Yellow Book).

Boivin and James showed the weight of timing in an experiment with a five-hour advance in the sleep-wake cycle. Ambient light received at the right time produced an advance of 5 hours and 22 minutes. Exposure at the wrong time kept the clock stuck to the departure time (Boivin and James, 2002). In other words, the same light can help or hinder.

How to recognize jet lag disorder

The CDC presents the criteria of the international classification of sleep disorders, third edition. For the diagnosis, the person must have insomnia or excessive daytime sleepiness, with reduced total sleep time, after a trip that crossed at least two time zones. Symptoms appear one to two days after the trip and come with impaired daytime functioning, general malaise or symptoms such as gastrointestinal ones. Finally, the picture cannot be better explained by another sleep disorder, another illness, medications or substance use (Riedy and Williams, CDC Yellow Book).

The captain meets the first two criteria easily. The third is the one that takes work. It forces the doctor to ask what else could explain two weeks of poor sleep, and that is where clinical reasoning comes in.

The direction of the flight changes the type of complaint

When the flight is eastbound, the local bedtime falls at a moment when the body clock is still in the middle of the afternoon or early evening. Sleep takes a while to come, and in the morning the person wakes up tired. When the flight is westbound, the pattern tends to be the opposite, with sleep too early and waking in the small hours. This reasoning comes from the physiology described by the CDC, which links the direction of the adjustment to the direction of the flight (Riedy and Williams, CDC Yellow Book).

Jet lag or sleep deprivation: the differential diagnosis

Criterion C makes clear that jet lag is a partial diagnosis of exclusion. The sources read for this text do not provide a complete list of other causes of insomnia, and so this post does not present one. What they do describe well are two explanations that compete in the captain's case.

Travel fatigue

Waterhouse and colleagues separate jet lag from travel fatigue. Both appear after long flights, but jet lag depends on the timing of the body clock in the new time zone (Waterhouse and colleagues, 2007). The consensus on athletes by Janse van Rensburg and colleagues also treats the two as distinct problems, with their own strategies before, during and after the flight, and acknowledges that evidence for strong recommendations is lacking (Janse van Rensburg and colleagues, 2021).

Accumulated sleep deprivation

The second explanation is sleep debt. Van Dongen and colleagues restricted the sleep of healthy adults for 14 days. Those who slept 6 hours or less per night had performance deficits equivalent to up to two full nights without sleep. The participants did not notice these deficits, and attention lapses were related to time awake beyond about 15.8 hours per day (Van Dongen and colleagues, 2003).

This finding weighs on the case. The captain says she is tired, but she may be underestimating how much. The study also shows that the subjective feeling is not a good measure of risk, and that applies to any professional who works shifts.

Why the two things get mixed up

In the crew, clock misalignment and lack of sleep usually come together. When bedtime falls at the wrong time on the clock, sleep is short. When sleep is short for days, the debt grows. The useful reasoning is not to pick a single explanation, but to measure both. The post about diagnostic uncertainty discusses how to decide when more than one hypothesis explains the picture.

Tests: the sleep diary comes first

The practice parameters of the American Academy of Sleep Medicine assessed the tests for circadian rhythm disorders. The sleep diary is recommended at the Guideline level. Actigraphy, which records movement with a device on the wrist, appears between Option and Guideline. Polysomnography is not indicated routinely, at the Standard level. Circadian phase markers are not recommended routinely for jet lag (Morgenthaler et al., 2007).

For the captain, this means a simple path. The first step is a sleep diary of a few weeks that includes the flight schedule, the time zones and the bedtime and wake times in each city. If the doctor wants more objective data, actigraphy can be added. The sources read do not define for how many weeks the diary should be kept.

What the diary needs to show

A useful diary for a crew member records more than bedtime. It needs to include the time on the base clock and on the local clock, naps, sleep on board, coffee and other stimulants, and the feeling of drowsiness at the start of each duty. With this, the doctor can see whether poor sleep follows eastward trips or also appears on days at home. The post about pretest probability helps to think about what each piece of data changes in the hypothesis.

Management: light, timing and melatonin

The CDC recommends a gradual adaptation when the trip allows it. Before departure, a person can shift bedtime by 1 hour per day, for 2 to 3 days, in the direction of the destination. For short trips, of less than two days, the CDC suggests keeping the home schedule instead of trying to adjust (Riedy and Williams, CDC Yellow Book).

Burgess et al. tested this adjustment before an eastward flight. In three days, advancing sleep by 1 hour per day, the onset of nighttime melatonin secretion advanced 0.6 hour with dim light, 1.5 hours with intermittent bright light in the morning and 2.1 hours with continuous bright light of more than three thousand lux (Burgess et al., 2003). The study shows that morning light speeds up the adjustment, but it was done in a laboratory, far from a real schedule.

Melatonin: dose and timing

The Cochrane review by Herxheimer and Petrie concluded that melatonin is remarkably effective for preventing or reducing jet lag. It should be taken near the intended bedtime at the destination, between 10 pm and midnight. The dose used for jet lag is described below, based on the full text from the CDC (Riedy and Williams, CDC Yellow Book).

The same review warns that melatonin taken at the wrong time, early in the day, causes drowsiness and delays adaptation. It calls for caution in people with epilepsy and in those using warfarin (Herxheimer and Petrie, 2002).

The CDC says that doses of 0.5 to 1 mg are often enough and that doses above 5 mg are not recommended (Riedy and Williams, CDC Yellow Book). It notes that, in the United States, melatonin is not regulated as a drug and the actual content of products may vary. Melatonin is ineffective if taken between midnight and 5 am on the biological clock. For eastward flights, the CDC reproduces military guidance to take it about 90 minutes before bedtime (Riedy and Williams, CDC Yellow Book). The American Academy of Sleep Medicine classifies melatonin for jet lag disorder at the Standard level (Morgenthaler et al., 2007).

Caffeine, naps and hypnotics

For daytime sleepiness, the CDC repeats the same military guidance: 200 mg of caffeine every 4 hours during the daylight period, stopping 6 hours before bedtime. The half-life of caffeine is about 5 hours, and that of modafinil about 12 hours, which helps explain why timing matters. The CDC also mentions naps of 20 to 30 minutes and asks people to avoid antihistamines, long-acting benzodiazepines and other long-acting benzodiazepine receptor agonists (Riedy and Williams, CDC Yellow Book).

The American Academy of Sleep Medicine parameters classify hypnotics for jet lag insomnia and stimulants for sleepiness as Option, a weaker recommendation level (Morgenthaler and colleagues, 2007). The sources read for this text do not define a hypnotic dose for jet lag, so this post cites none. For crew members, any medication that causes drowsiness or stimulation also falls under the rules of the medical examiner and the aviation authority of each country.

The flight schedule as part of the diagnosis

In the captain's case, the schedule is part of the clinical history. Sallinen and colleagues asked airline pilots why they felt sleepy on duty. The most cited reasons were the time of the duty period and insufficient sleep, especially on early morning and night duties (Sallinen and colleagues, 2021).

In an earlier study, the same group compared short and long flights. The duties that covered the entire night on the base clock, from midnight to 6 a.m., were the worst. In about one in three of them, the sleep-wake relationship was reduced, and in about one in two there was strong sleepiness, 8 to 9 on the Karolinska scale. The authors ask that alertness be assessed in all phases of flight, not only at the start of descent (Sallinen and colleagues, 2017).

Onboard rest and the overnight stay

Onboard rest helps, but it does not replace a night on the ground. Signal and colleagues measured crew members' sleep during a 7-hour rest break. Sleep efficiency was 70 out of 100 in the air, against 88 out of 100 on the ground, with more awakenings (Signal and colleagues, 2013). Gregory and colleagues compared the second and third rest breaks and concluded that the second was not inferior to the third for sleep, fatigue and sleepiness at landing (Gregory and colleagues, 2021).

The overnight stay also counts. Cosgrave and colleagues showed that the end time of the overnight stay, measured on the base clock, matters for sleep before the return flight. Sleep was longer on overnight stays that ended between noon and mid-afternoon at home time (Cosgrave and colleagues, 2018). These data help the doctor see the schedule as a series of timing exposures, not just as flight hours.

Reasoning: when the obvious explanation is not enough

The captain's case has an explanation that comes quickly: jet lag. It is likely, but it does not by itself explain two weeks of poor sleep after a four-day trip. Criterion C of the classification exists precisely to remind us that other causes may be behind it (Riedy and Williams, CDC Yellow Book).

Croskerry describes premature closure, the tendency to accept a diagnosis before confirming it (Croskerry, 2003). In the scenario, premature closure would mean ending the consultation with generic advice about melatonin. The post about the diagnostic pause shows how to stop and ask what has not yet been explained.

Chronic fatigue is also a matter of the professional's own health

Weeks of poor sleep in someone who works shifts comes close to a topic that medicine knows well among doctors themselves. The post about burnout in medical school covers the signs of exhaustion and prevention in people who live through long and irregular routines. The sources read for this text did not study burnout in crew members, so the post does not make that link in a clinical way. What remains is only the reminder to ask how the person is doing beyond sleep.

Case summary in a table

Case clue	What it suggests	What the source read says
Six time zones eastward	The body clock needs to move forward	Adaptation of about 1 hour per day eastward (CDC)
Takes a long time to fall asleep at 11 p.m.	Local time falls early on the body clock	Eastward is harder than westward (Herxheimer, Burgess)
Short sleep onboard	Sleep debt	Efficiency of 70 out of 100 in the air (Signal)
Two weeks of poor sleep	Accumulated sleep debt or another cause	Deficits without awareness (Van Dongen) and criterion C (CDC)
Overnight shifts	Sleep at the wrong time	Most cited cause of sleepiness (Sallinen)
Question about tests	Sleep diary first	Guideline for a diary, polysomnography not routine (Morgenthaler)
Question about melatonin	Low dose at the right time	0.5 to 1 mg is often enough; above 5 mg is not recommended (Cochrane review)

How NEXO helps train this reasoning

To use the case of the day as training, try three habits. Before each guess, ask what time the symptoms started and which time zone the person was in. When an explanation seems obvious, ask what it does not explain. After the result, note which timing clue you let slip.

Three exercises with cases

Draw the timeline. Mark sleep times, trips and symptoms on the same axis.

Separate the hypotheses. Write what each explanation predicts and which data would tell them apart.

Look for what is left over. List the findings the first hypothesis does not cover.

The post about learning based on clinical cases shows how cases help cement this habit.

The limits on using the game are described on the NEXO economy page. If you research the teaching of clinical reasoning, the page for researchers explains how to get in touch. Professionals who write or review cases find information under curation and creators, and institutions can learn about the proposal on the partnerships page.

Frequently asked questions

Is jet lag caused by the misalignment of the biological clock?

Yes. It happens when the body clock stays on the departure time while the person lives on the destination time.

Is flying east usually harder?

Yes. The human clock delays more easily than it advances, and flying east requires advancing it.

Is the sleep diary the first recommended test?

Yes. Sleep medicine guidelines recommend the diary and leave polysomnography out of routine practice.

Does melatonin work better near bedtime at the destination?

Yes. The Cochrane review showed a benefit when it is taken near the intended bedtime in the new time zone.

Does morning light help advance the clock?

Yes. Received after the lowest point of body temperature, it advances the clock and speeds up adjustment to the east.

Can lack of sleep worsen performance without the person noticing?

Yes. In a sleep restriction study, the participants did not notice their own deficits.

Final checklist

• I asked how many time zones the person crossed and in which direction.
• I recorded bedtime and wake time on the home clock and on the local clock.
• I checked whether the symptoms started one to two days after the trip.
• I asked about sleep on board and about naps.
• I considered accumulated sleep debt beyond jet lag.
• I remembered that criterion C calls for ruling out other explanations.
• I asked for a sleep diary before thinking about other tests.
• I discussed morning light and bedtime before medications.
• I reviewed the melatonin dose and timing and the cautions from the Cochrane review.
• I remembered that medications for crew members follow the examining physician and the aviation authority.

Want to practice reasoning with progressive clues? Play the case of the day.

References

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2. Burgess HJ, Crowley SJ, Gazda CJ, et al. Preflight adjustment to eastward travel: 3 days of advancing sleep with and without morning bright light. J Biol Rhythms. 2003. PMID 12932084.
3. Cosgrave J, Wu LJ, van den Berg M, et al. Sleep on Long Haul Layovers and Pilot Fatigue at the Start of the Next Duty Period. Aerosp Med Hum Perform. 2018. PMID 29233240.
4. Croskerry P. The importance of cognitive errors in diagnosis and strategies to minimize them. Acad Med. 2003. PMID 12915363.
5. Gregory KB, Soriano-Smith RN, Lamp ACM, et al. Flight Crew Alertness and Sleep Relative to Timing of In-Flight Rest Periods in Long-Haul Flights. Aerosp Med Hum Perform. 2021. PMID 33468288.
6. Herxheimer A. Jet lag. BMJ Clin Evid. 2014. PMID 24780537.
7. Herxheimer A, Petrie KJ. Melatonin for the prevention and treatment of jet lag. Cochrane Database Syst Rev. 2002. PMID 12076414.
8. Janse van Rensburg DC, Jansen van Rensburg A, Fowler PM, et al. Managing Travel Fatigue and Jet Lag in Athletes: A Review and Consensus Statement. Sports Med. 2021. PMID 34263388.
9. Morgenthaler TI, Lee-Chiong T, Alessi C, et al. Practice parameters for the clinical evaluation and treatment of circadian rhythm sleep disorders. An American Academy of Sleep Medicine report. Sleep. 2007. PMID 18041479.
10. Riedy SM, Williams SG. Jet Lag Disorder. CDC Yellow Book: Health Information for International Travel. Centers for Disease Control and Prevention, 2026. PMID 41818499. Document.
11. Sallinen M, Onninen J, Ketola K, et al. Self-reported reasons for on-duty sleepiness among commercial airline pilots. Chronobiol Int. 2021. PMID 33985396.
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