In for the long haul: the sleeping-competing connection

Understanding the circadian rhythm

The daily circadian rhythm is most profound, and best researched in the context of sport performance. As a result of the rhythmic fluctuations induced by the circadian rhythm, many people experience maximum mental alertness, fastest reaction times and highest core temperatures in the late afternoon/early evening period. By contrast, the peak in melatonin (the body’s sleep hormone) concentrations in the middle of the night period leads to maximum fatigue/sleepiness and lowest alertness at that time. The implication of these fluctuations for sportsmen and women is that concentration, skill learning, motor skill performance and muscular flexibility change throughout the day and this has indeed found to be the case. Evidence shows that these performance-related attributes tend to peak in the late afternoon/early evening, in line with core temperatures^(2-5).

As mentioned above, the circadian rhythm governs sleep patterns, which is perhaps not surprising since it is synchronized to the night-day cycle and the body’s internal clock, which actually runs at just over 24 hours. This body-clock effect is evidenced by the fact that if you remove someone from exposure to day-night stimuli (eg put them in a lab for a few days) and let them sleep and wake without superimposing a day-night rhythm on them, each day they will go to bed and wake up a bit later. Allowed to run freely the internal body clock is slightly more than 24 hours, but is kept in check by exposure to daylight and night time⁽⁶⁾.

Disturbances to the circadian rhythm

Given that the circadian rhythm is strongly influenced by day-night cycles, it follows that the circadian rhythm can be disturbed, something that unfortunately can be all too common in modern life! These disturbances may be minor – for example, exposure to bright light containing blue wavelengths (ie simulating daylight) by spending time on a phone or computer screen in the run up to bedtime. This practice delays the onset of sleep, which if repeated regularly disturbs the normal rhythm by pushing it back⁽⁷⁾.

The same is true for night shift work, which profoundly disrupts the normal circadian rhythm, potentially resulting in an array of negative health consequences for those who regularly undertake night shift work⁽⁸⁾. Indeed, such are the profound effects of repeated night-shift disruptions of circadian rhythm that ‘shift work intolerance syndrome’ is now recognised as a health condition in itself. This condition is characterized by chronic fatigue, sleep disorders, digestive system diseases, psychoneurotic ailments -all of which may appear after a few months or several years of shift work⁽⁹⁾.

Circadian rhythm and jetlag

Most athletes don’t do night shifts but something that may happen to desynchronize internal circadian rhythms is long haul air travel across three or more time zones. To give you an idea of the impact, consider an athlete who travels from Miami to compete in London (5 hours ahead). Upon departing Miami, he or she will have an internal circadian rhythm where peak performance occurs around 5pm/6pm in Miami, but which equates to 10pm/11pm London time - almost certainly NOT when any competition will be taking place! Moreover, a Miami athlete who heads to bed at 10.30pm London time will have an internal clock telling them it’s actually only 5.30pm, so will most likely struggle to fall asleep. If our athlete then needs to rise at 6.30am London time the following morning, not only will he/she likely to be sleep deprived, the internal body clock will be registering 1.30am! Unsurprisingly, this will leave the athlete feeling and performing out of sorts!

This temporary circadian misalignment and its associated consequences are technically known as ‘desynchronosis’, or more colloquially ‘jet lag’ – consequences that will last until the internal circadian rhythms adapt and synchronize to the time zone of the new destination. When it comes to athletic performance, the effects of jet lag can be very significant; the fatigue and circadian disruption due to a mismatching of internal biological rhythms with a new time zone have been clearly shown to be detrimental to athletic recovery and performance⁽¹⁰⁾. However, because the free-running circadian rhythm is a little longer than the 24-hour day, most people adapt more easily to a time zone change that involves later timings (westwards travel) than earlier timings (eastwards travel)⁽¹¹⁾.

Factors affecting jetlag

Jetlag induces a number of unpleasant effects (demonstrating just how important the circadian rhythm is). These include sleep disturbance, daytime sleepiness, difficulty sleeping at night, irritability, gastrointestinal disruption and reduced cognitive and physical performance⁽¹²⁾. Needless to say, these effects are hardly conducive to maximum athletic performance! As a rule of thumb, the greater the number of time zones crossed, the more travellers suffer jetlag.

A trip to Miami from Los Angeles involves crossing three timezones eastwards – ie your internal body clock has to reset to a time that is three hours earlier. However, flying from Los Angeles to London involves crossing eight timezones eastwards, requiring much more adjustment and resulting in significantly greater jetlag⁽¹³⁾. It is even reported that north-south long haul travel can also cause jet lag due to changes in day length even when no timezones are crossed⁽¹⁴⁾. For example, a December flight from Buenos Aires in Argentine to Newfoundland would mean transiting from midsummer where the sun rises at 6am and sets at 8pm to midwinter, where the sun rises at 8am and sets by 4pm!

Susceptibility to jetlag

If you’ve ever travelled with friends or family on a long-haul trip, you’ll already know that while jetlag can affect everyone, some people are more susceptible than others. But why is this? Research has identified a number of different explanations:

·        Owl/lark tendency – some individuals are naturally ‘morning’ people (larks), who find it easy to get up early and perform well early in the morning but struggle to stay awake late at night. Others are more owl like, struggling to get up and function in the morning but performing well in the evening and find staying up late easy – see this article on owls and larks. As a rule of thumb therefore, larks find it easier to cross timezones eastwards (where the internal clock needs to adjust to an earlier time) while owls find it easier to travel westwards, where the internal clock resets to a later time^(15,16).

·        Gender – Although it is not understood why, men tend to go to sleep earlier and experience less fatigue than women after crossing multiple time zones during eastward travel.

·        Direction of travel – as previously mentioned, jet lag is often more pronounced on eastbound trips than on westbound trips, where it is easier to stay awake than to go to bed early. However, your owl/lark tendency will also determine what you experience⁽¹⁷⁾.

·        Fitness – some research suggests that higher levels of physical fitness can help promote sleep in athletes as well as with mental toughness, which may improve athletes’ ability to cope with jet lag⁽¹⁸⁾. This might mean that amateur athletes travelling for competition need to allow more time for ‘resynchronization’ when arriving at the destination.

Accommodating and overcoming jetlag

How can athletes best manage and overcome jetlag? In particular, how much time is needed to fully overcome the performance-denting effects? The consensus of opinion is that the speed at which circadian rhythm resets itself is limited, and typically occurs at a rate of approximately 1 hour per day^(19,20). However, some research suggests that a more nuanced approach is needed, and that recovery takes place at a rate of approximately half a day per time zone crossed when travelling west and one day per time zone when travelling east⁽²¹⁾. This in turns means that athletes travelling west for competition will need less time to adapt (unless they are extreme larks!) but will need to factor in a longer recovery when travelling back home again after the event (important if you need to get back to work). Of course, the reverse is true when travelling east.

So what exactly can an athlete expect to experience when crossing multiple timezones? How much harder is it to get to sleep, how much sleep will be lost, how does the quality of sleep change, and how does that manifest itself in terms of what the athlete experiences? Despite a good body of evidence on the general principles and physiology of long-haul travel and jetlag, it turns out that there’s actually very little real-world data on athletes crossing timezones, and the impact on sleep habits and resulting fatigue. But now a new study on international track cyclists provides a fascinating insight.

New research

Appearing in the journal Neurobiology of Sleep and Circadian Rhythms, this study investigated the impact on elite track cyclists who undertook long-haul travel across eight timezones in an eastwards direction from Madrid to Hong Kong⁽²²⁾. In particular, the researchers wanted to assess the cyclists’ baseline sleep patterns, the impact of the long-haul travel on their subjective and objective sleep measures and how this then affected their sleep upon arrival at the destination.

An entire elite international track cycling squad was recruited consisting of for male and three female cyclists who had to travel from Mallorca (Spain) to Hong Kong. This involved three flights:

·        Flight 1: Majorca to Madrid (1 h 25 min). Transit: 2 hours.

·        Flight 2: Madrid to Dubai (7 h 15 min). Transit: 3 hours.

·        Flight 3: Dubai to Hong Kong (7 h 50 min).

·        Total travel time: 21.5 hours.

During the five days before the journey began, all the participants wore a sleep activity monitor (Philips Respironics ActiWatch 2) at night. They also wore the monitor during the long haul travel then for a further five day after arrival. In addition, the cyclists also completed a daily online sleep diary. A schematic outline is shown in figure 2.

Figure 2: Outline of sleep and long-haul travel study

The following measures of the cyclists’ sleep quality were derived from their actigraphy data:

·        Time in bed (min): the time spent in bed between going to bed and getting up.

·        Total sleep time (min): the amount of time spent asleep.

·        Sleep onset latency (min): the amount of time it took to get to sleep.

·        Sleep efficiency (%): sleep duration expressed as a percentage of time in bed.

·        Wake after sleep onset (min): time spent awake between start and end of sleep.

·        Awakenings: number of awakenings.

What did they find?

When the data from the sleep actigraphs was collected and analyzed, few key findings became apparent. Compared to the 5-day period prior to travel, the following occurred:

·        Time in bed was reduced.

·        The total sleep time per night reduced and the quality of the sleep that the cyclists did get was also reduced.

·        The amount of fatigue experienced at the end of each day increased significantly.

·        The time taken to get to sleep and the number of awakenings per night were not impacted by the travel.

·        The most significant sleep impact effects were seen during the 48-hour period after arriving in Hong Kong. However, as figure 3 shows, even after five days, the amount of high-quality sleep was still not restored to the pre-travel levels – specifically, no athletes experienced any hours of ‘very good’ quality sleep (see figure 3).

Figure 3: Sleep actigraph data pre, during and post long-haul travel

Vertical axis shows day of study, from five days prior (top) to five days post travel (bottom). Numbers within each bar show number of hours of each sleep quality per night. Note how ‘very good’ sleep quality vanishes and even ‘good’ sleep quality plummets for the first two days after arrival at destination and fails to fully recover, even after five days.

Implications for athletes

What do these and the other recent findings mean for the travelling athlete? Firstly, we can see just how severe the impact of crossing 8 time zones eastwards was for these cyclists. One hour of decent quality sleep per night will leave even the toughest and most resilient athlete feeling pretty tired and rubbish! Even after five days, none of the cyclists was experiencing any ‘very good’ sleep quality. Since adequate and good quality sleep is now known to be critical for optimum athletic performance – with between seven and nine hours per night recommended^(23,24) - this data certainly adds credence to the notion that at least one day per time zone is needed when flying eastwards.

The above study didn’t assess the owl/lark characteristics of the cyclists, but it’s likely that those who were natural owls would have been more severely impacted compared to larks, and could even require more than one day per time zone for optimum preparation. Also, looking at the magnitude of the sleep disruption, allowing just 30 minutes per time zone when flying westwards might be rather too ambitious; As the author of this article who has more lark than owl tendencies can personally attest, I found it more to adjust my body clock when flying six hours west from England to the US than flying six hours east to the Indian sub-continent!

Let summarize some practical recommendations for athletes who are intending to undertake long-haul travel to a forthcoming competition or event:

·        Allow at least day per time zone (hour) for circadian rhythm adjustment when travelling across zones, especially when travelling east.

·        Owls may need to allow more than one day per time zone when travelling east; larks may need more than one day per time zone when travelling west.

·        When travelling eastward, female athletes may need more time to adapt then male athletes.

·        The above recommendations are derived from elite athletes who are thought to adapt more rapidly than amateur athletes. If you’re an amateur athlete, you should consider allowing a little more time – perhaps 1.25 to 1.5 days per time zone.

·        All some adaptation time when undertaking long haul travel from north to south and vice-versa. Even though less than three time zones may be crossed, when travel takes place in midsummer or midwinter, the sudden change in day length may disturb your circadian rhythm.

·        When travelling west, research suggests that late evening bright light and exercise can also help delay and resyncronize circadian rhythm⁽²⁵⁾.

·        When travelling east or preparing yourself for a very early competition, early-morning bright light is helpful for resetting circadian rhythm, and can be performed in advance (ie prior to the trip) using several smaller steps of 30 minutes over a longer period, rather than fewer but larger steps⁽²⁶⁾.

·        Following good general sleep hygiene practices will help reduce sleep disturbance – or at least not exacerbate it following long-haul travel. This means ensuring your room temperature is conducive to sleep (around 18C (64F) is ideal), avoiding computer and phone screens in the hour before retiring, and not consuming caffeine for at least six hours before retiring!

 References

1.       Lancet 2001. 358: 999-1005

2.       Int J Sports Med. 1997 Oct;18(7):538-42

3.       Eur J Appl Physiol. 2004 Jun;92(1-2):69-74

4.       Can J Sport Sci. 1992 Dec;17(4):316-9

5.       Int J Sports Med. 2004 Jan;25(1):14-9

6.       Science 1999; 284 (5423), 2177–2181

7.       Encephale. 2018 Sep;44(4):321-328

8.       Int J Environ Res Public Health. 2022 Aug; 19(16): 9802

9.       Psychiatr. Pol. 2017;51:815–832

10.     J. Clin. Sleep Med. 2021; 17 (2), 243–248

11.     J. Biol. Rhythms. 2003 18 (4), 318–328

12.     Clin. J. Sport Med. 2012;22(3):68–273

13.     Aviat Space Environ. Med. 2003;74:173–179

14.     J. Theor. Biol. 2018;437:261–285

15.     Trav. Med. Infect. Dis. 2009;7(2):88–101

16.     Sleep Res. 2000;9(2):117–127

17.     Br. J. Sports Med. 2002;36(1):54–60

18.     Med. Sci. Sports Exerc. 2017;49(12):2548–2561

19.     Proc. Natl. Acad. Sci. U.S.A. 2021; 114 (6), 1407–1412

20.     Psychiatry Clin. Neurosci. 1999; 53 (2), 257–260

21.     Aspetar Sports Med. J. 2019;8:214–222

22.     Neurobiol Sleep Circadian Rhythms. 2023 Nov; 15: 100102

23.     Sleep Health. 2015;1(1):40–43

24.     Int. J. Sports Physiol. Perform. 2021;1(aop):1–12

25.     Am J Physiol Regul Integr Comp Physiol. 2004 Jun;286(6):R1077-84

26.     Aviat Space Environ Med. 2006 Jul;77(7):677-86