– Written by Audrey Jansen van Rensburg, South Africa, David Stevens, Australia, Antonia Rossiter, Ireland, and Dina C. (Christa) Janse van Rensburg, South Africa
INTRODUCTION
An elite track and field athlete’s travel calendar is jam-packed throughout the year, with athletes required to travel worldwide for training camps in specific locations (heat or altitude training) or to compete in various local, regional, national, and international competitions1-3. The pinnacle of the track and field calendar includes high-profile international events, such as the Olympics, World Championships, and the Diamond League, which holds events in Asia, the Middle East, Europe, and North America4. For athletes to be successful and to deliver exceptional performances at such events, they need to be in peak physical and mental condition.5 However, these athletes frequently deal with the stresses of long-haul transmeridian air travel, such as crossing borders, adjusting to new time zones, sleep schedule disruptions, restricted movement and striving to deliver exceptional performances1-3. They must carefully manage their training, recovery, and competition schedule to perform at their best. The demands of long-haul transmeridian air travel across multiple time zones manifest as travel fatigue and jet lag. It can impose a significant physical and emotional burden on athletes, potentially impacting performance negatively. It is, therefore, critical to address this issue effectively. A recent consensus statement to help athletes manage travel fatigue and jet lag offers a few guiding principles1.
OUR BODY'S CIRCADIAN RHYTHM
The circadian rhythm is a natural, internal process that regulates our sleep-wake cycle and various physiological and behavioural patterns over 24 hours. The body’s master clock, a group of nerve cells (neurons) that form the suprachiasmatic nucleus (SCN), is located in the brain’s hypothalamus and is synchronised by a sophisticated system6. The master clock endogenously coordinates these rhythms.
The circadian rhythm synchronises various intrinsic biological processes and aligns our physiological and behavioural functions with the natural external 24-hour day-night cycle7. This includes regulating cognitive performance, sleep patterns, blood pressure, heart rate, body temperature, hormone production, metabolic processes, and other essential processes7-8.
The clocks of the circadian system synchronise through zeitgebers or time-givers9. The most important are light and darkness, which help to adjust the body’s internal clock to match the time of day10. The retina’s light-sensitive cells receive light directly and control the sleep-wake cycle by suppressing melatonin hormone production that peaks during the dark phase10-11. Other zeitgebers include physical activity, meals, and social interaction8.
Biologically, we all have an intrinsic chronotype called morning, intermediate, and evening types. Morning type individuals naturally tend to complete activities in the morning and go to sleep early. Evening types choose to complete activities in the evening and delay sleep12. An intermediate chronotype is the most common amongst adults12. Our chronotype or diurnal preference (morning versus evening type) also correlates to our endogenous circadian rhythms of core body temperature and melatonin levels12-13.
The levels of endogenous melatonin, a sleep-promoting hormone, follow a circadian pattern, and activity occurs exclusively at night. It rises in preparation for sleep, reaching its highest peak during sleep, decreasing toward the early morning hours, and remaining low until nighttime14. Melatonin is secreted daily under dim light conditions and typically occurs 2-3 hours before the habitual onset of nocturnal sleep, referred to as the dim light melatonin onset (DLMO) point1,15.
Cortisol is best known as the body’s stress hormone and is crucial in managing our sleep “architecture”. Cortisol follows a specific circadian pattern. Its levels are typically high in the morning, assisting in our wakefulness, and gradually decreases throughout the day, reaching its lowest point during nighttime sleep. A blunted cortisol awakening response (a change in cortisol concentration occurring in the first hour after awakening from sleep) can indicate various problems, including physical and mental health issues16.
During the night, as our body enters its habitual sleep phase, our core body temperature gradually decreases. Core body temperature minimum (CBTmin) is the lowest point that an individual’s core body temperature reaches during a standard 24-hour cycle. This minimum core body temperature typically occurs during the early morning hours, around 4:00am - 6:00am, just before awakening. It is also when our body’s metabolic rate is at its lowest and associated with the time of deepest sleep. As morning approaches and the body starts transitioning to wakefulness, core body temperature rises. The daily peak of our core body temperature peaks (CBTmax) is approximately 12 hours after CBTmin1,15.
Our circadian rhythms affect our state of wakefulness and play a significant role in regulating alertness. These natural fluctuations in alertness and wakefulness occur over a 24-hour period and tend to align with natural daylight. As a result, we experience increased daytime and decreased nighttime alertness. Disruptions to these rhythms can lead to decreased cognitive performance and impaired reaction time17 (see Figure 1).
The two most robust indicators of the circadian phase are the DLMO and CBTmin time points. These two markers, DLMO and CBTmin, play a crucial role in managing activities based on the timing of our internal circadian phase to reduce the discomfort and disorientation associated with jet lag1.
THE DIFFERENCE BETWEEN TRAVEL FATIGUE AND JET LAG
While travel fatigue can result from any type of lengthy travel, jet lag follows east- or westward travel across multiple time zones. It explicitly involves disrupting the body’s internal clock. Travel fatigue and jet lag consider the travel duration, travel direction, the number of time zones crossed, the frequency of journeys, and the duration of the competition season1. The anticipation of travel and dealing with unfamiliar environments and cultural differences can contribute to heightened anxiety levels, especially in individuals predisposed to travel-related stress. Athletes travelling for competition have the additional pressure of having to perform after arrival at their destination. These stressors can compound the effects of travel fatigue and jet lag.
Travel fatigue often resolves following a recuperative nap or relaxation, a refreshing shower or a good night’s sleep1,3. The severity and duration of jet lag symptoms adjust gradually and over time and persist until the circadian rhythm shifts to the time of the new environment1,3,18.
Travel Fatigue and Its Impact on Athletes
Travel fatigue is temporary exhaustion and a general feeling of weariness and tiredness that follows any long journey, including car, bus and train trips. It can accumulate throughout a season due to a congested competition schedule, high training loads and poor recovery. Travel fatigue in isolation, involves national or international trips crossing less than 3 time zones and travelling translatitudinal (environmental changes, i.e. winter-summer/summer-winter). Specific elements related to air travel may further contribute to travel fatigue. These include factors such as the average flight altitude, cabin oxygen saturation levels, air pressure changes, seating conditions and the frequency of travel across a season. All of these contribute to feeling exhausted after a long flight.
Travel fatigue develops from prolonged inactivity, extended periods of sitting, disrupted sleep patterns, restricted food choices, dehydration, hypoxic environment and other stresses associated with long-distance travel. It is characterised by persistent fatigue, changes in behaviour and mood, loss of motivation, reduced performance, recurring illness and increased injury occurrence1,3. Symptoms may also include tiredness and headaches and feeling dehydrated, bloated and swollen1.
Travel fatigue is a multifaceted phenomenon associated with decreased physical and cognitive performance. Physically, the strain of travel can lead to reduced muscular strength, endurance, and coordination, affecting athletes’ ability to engage in activities effectively. Cognitive abilities, including decision-making, attention span, and reaction time, can also be compromised, potentially hindering athletes from navigating the new environment and situations19.
To detect and correct these consequences requires ongoing monitoring of the athlete, including the adjustment to training, travel and competition loads. The importance of monitoring tools for assessing travel fatigue, especially cumulative travel fatigue, cannot be overstated. Without validated methods to track this condition, the effective implementation of recovery strategies and the adoption of healthy sleep practices become challenging1,3.
A travel fatigue scenario (crossing <3 time zones)
Setting: Elite track and field athletes travelling from South Africa to Paris to participate in the 2024 Summer Olympic Games. Athletes will travel 12 554km from the Southern to the Northern Hemisphere during a 10-hour and 40-minute flight, crossing only 2 time zones and a season change.
This journey is likely to cause acute travel fatigue but not jet lag. Immediately after the flight, the athlete’s CBTmin will occur at 04h00 destination time, the same as home time. If, in the lead-up to the Olympics, these athletes experience a tight schedule due to high training and competition loads and inadequate recovery time, they will suffer from cumulative travel fatigue. Travelling from the northern hemisphere to the southern hemisphere, or the opposite direction, can have several effects due to the change in geographical location. These effects can include adjustments to the change in seasons, climate, and daylight hours. Changes in climate and environment can sometimes affect our health, so it’s essential to be aware of potential health risks associated with the destination, including exposure to different allergens or diseases1 (see Figure 2).
Jet Lag and Its Impact on Athletes
Jet lag is a specific type of travel-related fatigue that follows a misalignment of an individual’s internal circadian rhythm with the new destination’s time zone. Rapid transmeridian travel (i.e. in the direction East–West/West–East) primarily causes jet lag. Crossing 3 or more time zones disrupts the body’s natural sleep-wake cycle, which causes jet lag. Jet lag is a temporary impairment, and symptoms may include1,3:
day-night confusion and sleep disturbances, finding it difficult to fall asleep and stay awake during the local daytime hours, fragmented sleep, early waking, or difficulty staying asleep during the night;
fatigue and irritability, experiencing extreme tiredness, irritability, and difficulty concentrating;
digestive disturbances such as nausea and indigestion as a result of changes in meal times, eating patterns and other biological functions;
physical discomfort such as headaches, muscle aches, and general distress;
and increased risk of illness.
Depending on the travel direction, the circadian system must delay (when travelling West) or advance (when travelling East). Additionally, the number of time zones crossed can influence the severity and duration of jet lag. Eastward travel is generally more challenging as endogenous circadian rhythms have a ~24-hour period, making it harder to advance than delay your circadian system. Typically, the timeline for natural circadian re-alignment takes 0.5 days per time zone crossed when travelling West (or 2 hours per day) and 1 day per time zone crossed when travelling East (or 1 hour per day)1,15. The daily time frame for implementation of interventions must also be delayed/advanced 1 hour each day until the circadian rhythm is synchronised1. Adjustments should include changes in seasons, climate, and daylight savings hours.
A combined jet lag and travel fatigue scenario (crossing >3 time zones WEST)
Elite track and field athletes travelling West from Sydney, Australia, to Paris to participate in the 2024 Summer Olympic Games. Athletes will cross from the southern to the northern hemisphere and travel close to 17 000km for at least 20 hours, including stopovers and crossing approximately 7 time zones.
These athletes will suffer from both travel fatigue and jet lag. Immediately after the flight, the athlete’s CBTmin will occur at 21h00 destination time instead of 04h00. Flying West delays the day, and athletes must shift their body clock backwards to an earlier time. These athletes will also experience a change in season (winter/summer), climate (cold/warm), and daylight hours. Resynchronisation is easier when flying West. It takes about half a day per time zone crossed and delays 2 hours daily. At approximately day 4, complete adaptation occurs.
A combined jet lag and travel fatigue scenario (crossing >3 time zones EAST)
Setting: Elite track and field athletes travelling East from Eugene, Oregon in the USA to Paris to participate in the 2024 Summer Olympic Games. Athletes will travel over 8 000km, crossing approximately 7 time zones, multiple flights, and even road travel.
These athletes will suffer from both travel fatigue and jet lag. Immediately after the flight, the athlete’s CBTmin will occur at 11h00 destination time instead of 04h00. Flying East advances the day, and athletes need to shift their body clock forward to a later time. Travelling East leads to more severe jet lag that is longer-lasting. Resynchronisation takes about 1 day per time zone crossed and advances 1 hour daily. At approximately day 7, complete adaptation occurs.
Strategies to delay/advance circadian rhythm using sunlight
Light, especially natural sunlight, is the circadian system’s primary zeitgeber and strongest synchroniser. The phase-resetting capacities to light mainly depend on the direction of travel, the number of time zones crossed, time of day, light intensity, and spectral composition. Implementing light exposure/avoidance strategies to adapt to the new time zone gradually can help alleviate jet lag effects1,20 (see Figure 3).
Travelling Westward
Crossing 4 time zones: aim to get up to 3 hours of natural light exposure, ideally outdoors, in the late evening and avoid light in the early morning to support phase delay.
Crossing 8 time zones: aim for natural light exposure in the early evening, avoiding it for up to 3 hours in the late evening.
Crossing 12 time zones: consider light exposure in the early to mid-afternoon and follow this with up to 3 hours of light avoidance in the early evening.
Travelling Eastward
Crossing 4 time zones: aim to get up to 3 hours of natural light exposure, ideally outdoors, in the mid-morning and avoid light in the earlier waking hours of the morning to support phase-advance.
Crossing 8 time zones: aim to get light exposure in the early afternoon and avoid it for up to 3 hours mid to late morning.
Crossing 12 time zones: follow the same pattern as when travelling Westward, crossing more than 12 time zones.
If light exposure is scheduled during the daytime, spending time outdoors in natural sunlight without wearing sunglasses is recommended. If light exposure is scheduled after sunset, it is advisable to utilise sources of bright indoor lighting, a lightbox, or light-emitting glasses. If light avoidance is scheduled during the daytime, staying indoors with the lights turned off or as dim as possible is recommended1.
Arriving at your destination with enough time to fully adjust to the new time zone is recommended. Research suggests that athletes travelling across multiple meridians in an eastward direction across eight time zones for major competitions should be allowed ten days to recover from jet lag and travel fatigue21.
PRACTICAL TIPS TO HELP MANAGE TRAVEL FATIGUE AND JET LAG IN ATHLETES
Managing travel fatigue and jet lag in athletes is essential to ensure performance and well-being during competitions. To alleviate travel fatigue and jet lag symptoms, athletes can adopt various strategies to help them navigate these challenges1 (see Figure 4).
ADDITIONAL TIPS TO ALLEVIATE TRAVEL FATIGUE AND JET LAG IN ATHLETES ON ARRIVAL AT THE DESTINATION
Upon reaching your destination, consider taking a refreshing shower to invigorate your senses. Allow time for gradual acclimatisation. Expect that performance might be slightly affected initially due to the travel-related challenges (see Table 1).
CONCLUSION
Most evidence concerning travel fatigue and jet lag management stems from non-athletic populations in laboratory studies. Interventions and strategies to lessen travel fatigue and jet lag commonly promoted include the selected and timeous use of light exposure/avoidance, sleep, exercise, nutrition, melatonin, stimulants and sedatives. The suitable application and timing of these interventions depend on the number of time zones crossed, the travel direction, the adaption period, and the individual’s chronotype. Due to the distinct nature of each athlete, it becomes crucial to tailor these strategies to individual preferences and needs. Current strategies encompass available evidence, general approaches and practical tips. Maintaining consistent routines, practising patience, and employing strategic planning to manage travel fatigue and jet lag may enable athletes to perform at their best despite demanding travel schedules.
Audrey Jansen van Rensburg B.Sc. (Hons), M.Sc., Ph.D.
Sport Exercise Researcher
Section Sports Medicine
University of Pretoria
Pretoria, South Africa
David Stevens B.App.Sc (Hons), Ph.D.
Adjunct Research Fellow,
Flinders Health and Medical Research Institute Sleep Health,
Flinders University,
Australia
Extraordinary Lecturer,
Section Sports Medicine
University of Pretoria
Pretoria, South Africa
Antonia Rossiter B.Sc. (Hons), Ph.D.
Performance Physiologist
Sport Ireland Institute
National Sports Campus
Dublin, Ireland
Sport and Human Performance Research Centre
Health Research Institute
University of Limerick
Limerick, Ireland
Dina C. (Christa) Janse van Rensburg M.B.Ch.B., M.Sc., M.Med., D.Med. (Ph.D.)
Specialist in Physical Medicine and Rheumatology
Professor in Sports and Exercise Medicine
Section Sports Medicine
University of Pretoria
Pretoria, South Africa
Chair of the Medical Advisory Panel
World Netball
Manchester, United Kingdom
Contact:
christa.jansevanrensburg@up.ac.za
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