Jet Lag and Travel Fatigue in Athletes

The body runs on circadian rhythms, internal cycles of roughly 24 hours that govern sleep, alertness, hormones and body temperature, coordinated by a master clock in the suprachiasmatic nucleus (SCN) of the hypothalamus and set mainly by light reaching the eye. In darkness the pineal gland releases melatonin, which signals biological night.

When several time zones are crossed quickly, the internal clock stays on home time while the world runs on destination time, and the two are misaligned until the clock resets. Adaptation is usually slower after travelling east, around an hour a day, and faster after travelling west, around two hours a day, with considerable individual variation. Travelling east needs the clock to move earlier, a phase advance, which is harder, while travelling west needs a phase delay, which is easier. The more zones crossed, the greater the disruption, and travel that crosses none, such as north to south, causes travel fatigue but not jet lag.

Jet lag typically disturbs sleep, with difficulty falling asleep, early waking or daytime sleepiness, alongside impaired concentration and decision-making, fatigue, irritability or low mood, and sometimes gastrointestinal upset. Symptoms are usually worst in the first few days and settle as the clock re-entrains. Travel fatigue overlaps, with tiredness, low motivation, disturbed sleep and a run-down feeling, but it comes from the journey and the disrupted routine rather than clock misalignment, and it can accumulate with repeated travel. For the athlete, both can slow reactions and reduce skill, endurance, power and recovery, though the effect varies between individuals.

Assessment is clinical and based on the history: the time zones crossed and the direction, the travel and sleep history, the symptom timeline and the competition schedule. The main task is to separate jet lag from travel fatigue, since the emphasis of management differs, and to consider an alternative diagnosis if the picture is atypical, severe or prolonged. No specific tests are needed, though a simple sleep and travel diary helps frequent travellers, and knowing the athlete's chronotype, whether they are a natural early or late type, helps individualise advice.

The timing of light is the most effective way to shift the body clock, but it is not a simple blanket rule, since its effect depends on biological time, the number of zones crossed and the phase response curve (PRC), which maps how light at different internal times advances or delays the clock. As a general guide, after travelling east the aim is to seek light later in the morning and avoid it in the late evening, and after travelling west to seek evening light and avoid early morning light.

Large eastward shifts, roughly more than six to eight time zones, carry a trap. The core body temperature minimum (Tmin), about a couple of hours before usual waking, is the pivot of the curve: light after it advances the clock, but light before it delays the clock. Early destination morning light can fall before the traveller's home Tmin and push the clock the wrong way, so for big eastward shifts the athlete should avoid bright light early in the destination morning, with sunglasses or by staying indoors, and seek it only later once the Tmin has passed. Large shifts therefore need an individual plan.

Melatonin can help the clock re-entrain and improve sleep, taken at the destination bedtime, and is most useful for eastward travel across several zones. In the UK some immediate-release melatonin products are licensed for the short-term treatment of jet lag in adults and are prescription-only, so it is used under medical advice, with attention to correct timing, a short duration and the usual contraindications, interactions and side effects. It also helps to adopt local time at once for meals, sleep and training, use short naps, shift sleep and wake times towards destination time before an important competition, use caffeine strategically and away from bedtime, and avoid alcohol. For a very short trip, staying closer to home time may be better.

Travel is planned so the team arrives with enough time to adapt before competing, allowing more for eastward travel and for larger time-zone shifts. During long journeys, moving regularly and doing calf exercises helps venous return, and an individual assessment is worthwhile for anyone with additional risk factors for a clot. Good hydration supports general health and performance, but it should not be presented as preventing thrombosis.

Sedative and hypnotic sleeping medicines, such as the Z-drugs and benzodiazepines, are best avoided during flights. Drug-induced deep sleep in a cramped, immobile seat increases the risk of deep vein thrombosis (DVT) and venous thromboembolism (VTE), sedation impairs alertness and coordination if an in-flight emergency arises, and residual grogginess on arrival impairs coordination, reaction time and training, while also complicating anti-doping testing even though these drugs are not prohibited by the World Anti-Doping Agency (WADA). If they are used at all, they are restricted to the short-term management of severe insomnia at the destination under close medical supervision.

Sleep is protected overall, prioritising adequate total sleep rather than following an aircraft schedule so rigidly that it causes further sleep loss. On arrival, the priorities are to get onto local time, use daylight and light activity, then protect sleep, monitor training load and ease back into full training. Across a season, travel is best built into preparation, with athletes monitored for accumulating fatigue.

• •Expert consensus on air travel and the elite athlete, from sports institutes and governing bodies, sets out light, sleep and melatonin strategies for crossing time zones.
• •Circadian science, including the phase response curve and the core body temperature minimum, underpins the direction-dependent and shift-dependent use of light.
• •Current UK prescribing guidance covers the licensed use and prescription-only status of melatonin.
• •UK Anti-Doping guidance advises checking any medicine on Global Drug Reference Online (Global DRO), while supplements need a need-risk-consequence assessment and, ideally, exact-batch testing, since a prohibited substance may not appear on the label.

• •Jet lag is clock misalignment from crossing time zones; travel fatigue is the toll of the journey and needs no zones crossed.
• •Eastward travel is a phase advance, harder and slower to adapt, about an hour a day, than westward, a phase delay, about two hours a day.
• •Light is powerful but not a blanket rule; large eastward shifts risk early morning light falling before the core temperature minimum and delaying the clock.
• •Melatonin at destination bedtime can help, especially eastward, and is prescription-only in the UK.
• •Avoid sedative sleeping tablets during flights, given the thrombosis and safety risks, and restrict them to severe insomnia at the destination if used at all.
• •After prolonged travel, watch for clot red flags such as one-sided leg swelling or breathlessness.